What Slack's invite flow teaches us about virality

A closer look at the mechanics behind Slack's PLG growth loop

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Slack was the fastest startup to hit a $1 billion valuation after launching in 2013.

Today, it has over 42 million daily active users — including 77% of Fortune 100 companies — with paid users spending about 90 minutes actively working in Slack.

Huge volumes. High engagement. And a product that’s expanded well beyond self-serve PLG into enterprise B2B (i.e. big boi companies).

The dream.

Slack isn’t just one of the earliest PLG stories - it’s one of the most iconic. And there's still loads to learn from it.

Especially when it comes to spinning referral loops faster and more efficiently.

This week’s deep dive breaks down the small UX decisions that drive a very well-oiled growth loop.

We’ll look at:

1. How to optimise referral loops with the K-factor
2. The UX behind Slack’s invite-sending flow
3. What the invitee (new user) experience looks like

Without further ado, the formula that underpins it all.

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PLG basics: introducing the K-Factor

PLG, sharing, referrals, virality — these concepts often overlap. Don’t get too caught up in the terminology. Focus instead on what actually drives growth.

One particularly useful concept is that of the K-Factor. It measures the number of new users a single existing user brings into the product.​

In formula terms:

In numbers, what this means is that:

• If your K-Factor is <1, your viral coefficient is low. Each user brings in fewer than one new user. Growth from this channel won’t scale.
• If your K-Factor is >1, you start seeing compounding growth.

There are three core levers that influence this:

1. % of users that share
2. How often users share (i.e. invites per user)
3. Conversion from viewed share link to sign up (invite acceptance rate)

These levers all work together to drive the PLG loop of a product.

The key is to track each step of the journey to spot where users drop off and focus your efforts where it matters most.

We’re going to look at how Slack optimises the UX for the two stakeholders in the share loop:

• The sharer experience (how easy it is to invite others)
• The invitee experience (what new users see when they’re invited)

Starting at the beginning, with sending an invite.

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Part 1 of 2: The sharer experience

I was playing around in Slack and realised I hadn’t tested the invite flow in a while. So I hit the three dots and jumped into settings.

What struck me was how high up sharing sits in the settings. First option on the list.

Notice how it's personalised:

It is also separated in its own section with two grey lines to give it more visibility and attention.

I click on ‘invite’ and get a pop-up share panel in the centre of my desktop.

Again, the panel’s title is personalised with the workspace name (useful for me, given I’m in eight workspaces).

Notice how little copy there is overall. Instead of a lengthly box title, the default copy helps me understand what I should do:

There’s also some helpful copy telling me what certain options are, like the ‘Member.’

The bit I like most is the ✨ customisation ✨

If I tap the blue hyperlink next to the magic wand, it expands to an extra two modules: channels and custom message.

In the custom message section, I see another example of great UX writing:

I type something in this section as a test to see where it shows up in the invite.

Once I hit send, I get an excellent confirmation page that covers:

• How many people I’ve invited
• Who I’ve sent the invite to
• Their role in the workspace
• When the invite expires

I love this because it covers off any fears or anxieties I have in this step, like:

• Is this invite going to expire?
• Did I type their email correctly?
• Will they have the right access?
• Hope I didn’t accidentally send more than one

As the person sending the invite, I’m feeling calm, content and chill.

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Part 2 of 2: The sharee experience

The first time I tested the flow, I went ahead with a non-customised share invite via SMS.

With links, what’s key is that the metadata is filled in.

Metadata for share links is behind-the-scenes info (like title, image, and description) that platforms like Slack use to generate link previews.

Why this matters:

• If you don’t control this metadata, your share links can look broken, blank, or unengaging
• If you don’t customise this for each platform, the invite won’t look native or ‘real’
• If you do control it, you can make shared content visually appealing, clear, and clickable — meaning a higher chance you’ll convert the invitee

Notice how Slack’s metadata is:

• Personalised with the workspace name
• Very casual, as if I’ve written the text
• Visually attractive with some UI illustrations that show me what to expect.

The email invite is also incredibly well-executed.

Notice:

• The subject line is personalised with the name of the referrer
• The header is personalised with the name of the referrer
• There are real faces of group members as little avatars, making the email come alive
• The number of people in the group is listed: 294 (as social proof 🤙)

And, this time, I filled in the message, which adds a personal touch.

So compelling, but also so compact. The design here is really impressive.

It’s all in the top of the fold, and without my customised message the CTA ‘join Slack’ would be visible without a scroll too.

I click on the CTA and get to a webpage which has the same building blocks of the share invite:

• Personalised header with [workplace name]
• Real avatars from the group
• Group size: 294
• Members’ names, like Rosie Hoggmascall and Hannah

Another detail that’s important is the two small bits of copy:

And:

The first is useful for product marketing, answering the question: What does this product do?

The second is useful for gathering email addresses that the data team at Slack can use to work out which companies are on Slack, and whether they’re worth upselling to.

This is called: Product-Led Sales.

I.e. the product is sending the sales team leads, who then go on to close bigger enterprise clients.

Why can’t those people convert themselves?

Well, in an interview on Lenny’s Podcast, Elena Verna shared:

She goes onto explain that after that, the sales team needs to come in to upsell above that amount and communicate enterprise value.

To do this effectively, data about product usage is used to direct the sales teams to the right users and accounts who are likely to upgrade.

BUT, without work email addresses, this is so much harder. Hence, Slack is making sure that people put in their work emails ahead of time in order to upsell to them later (if relevant).

And this is why 77 out of 100 of the Fortune 100 companies now use Slack.

(I’d highly recommend Elena’s substack on B2B growth, my current go-to resource.)

Back onto the flow, and last but not least, we have the launch screen, which again is simple, personalised and has some excellent UX writing on it.

I get a pop-up from my browser asking if I’d like to open Slack, which I click. Whilst I’m waiting, I can see some simple instructions and troubleshooting, with the option to use Slack in my browser.

What’s important in this invitee experience is how the UX flows between touch points: the branding, the messaging, the personalisation and more.

There’s no jarring transitions, no different fonts, instead it all feels like part of the same journey.

This is super hard when you’re moving between platforms yet Slack seems to effortlessly move from email to web page to desktop.

It felt easy, but execution-wise, that’s really difficult.

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What does this teach us about PLG loops?

First, it’s worth noting that sharing is core to Slack’s business. You have to work with others. And each extra teammate on the platform improves the experience for every other teammate.

This means Slack has a natural advantage when it comes to virality and referral loops — it’s structurally easier for them to achieve a K-Factor above 1.

Even if your product isn’t inherently social, there are still powerful lessons here for how to design better share flows:

1. Build for both sides of the share loop: growth doesn’t just depend on how many invites are sent. It depends on how many are accepted. Slack pays just as much attention to the invitee experience as the referrer’s. Most companies don’t.
2. Put share CTAs in high-traffic areas: the easier a button is to find, the more clicks it gets (as a general rule). Slack puts ‘invite’ as the top option in settings, as high as it can be.
3. Reduce fears for the sharer: The invite flow reassures users at every step: who they’ve invited, what role they gave them, and when the invite expires. The more secure a user feels in their action, the more likely they are to convert.
4. Use defaults to speed users through: small details like well-thought-through default copy can help users get through an experience quicker, all of which are helpful in improving conversion to share.
5. Personalise as much as possible: From workspace names to real user avatars, Slack bakes personalisation into its metadata, email invites, and landing pages. It feels like a message from a teammate — not a tool.
6. Use an intermediary web screen: you don’t have to have an intermediary screen; many share links just open the app (especially on mobile). Slack’s choice to actually build in a journey between invite and acceptance builds confidence, sets expectations, captures useful data (like work emails), and handles friction. It’s a growth lever, not just a few loading screens.
7. Think of share flows as retention moments: Sharing isn’t just about acquisition. It’s a moment to delight your existing users. Slack treats this as a full product experience, not just something that’s functional.

In short, virality isn’t magic. It’s a matter of structure, UX and psychology working together. Slack’s share loop is a lesson in how micro details — placement, copy, personalisation — combine to keep the loop spinning.

Next time you’re designing a share flow, ask yourself: would this make me feel confident to invite my team, friends or family?

Wooho! This was a fun one to write - funnily enough I meant to do a 60-second read this week (as it's a short week with the Easter bank holiday here in the UK), however I ended up writing 1,800 words. Oh well.

If you enjoyed this - share with someone you know so we can get the Growth Dives referral loops goin' ⚙️

See you next week (where I might actually succeed in writing a quick dive for once),

Rosie 🕺

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