How Slack got its users

Make a person exist in your product before they have an account. Slack calls them invited members: someone who was sent an invitation and has never signed in, but who can still be added to channels, mentioned, and sent DMs, and who is not billed on paid plans until they become active. Build the same object, a participant identified by nothing but an email address, so an existing user can hand them work today instead of asking them to sign up first.

done whenIn your database, can a task, mention, or comment be assigned to an email address with no user row behind it? If assigning anything requires an existing account, your invite is a marketing email, not a work dependency.

Put the work inside the notification email and accept the reply back. Slack's email to an invited member carries the message itself, and when that person hits reply from their inbox, Slack posts it into the right channel, thread, or DM. Set up an inbound mail address on your own domain, parse the reply, and post it as that person's message so everyone else in the room sees a non-user participating like a member.

done whenCan someone who has never opened your app produce content that your existing users see, using only their email client? If the only button in your notification is Sign up to view, you have a wall where the loop should be.

Scope the leak to the email domain so it crawls desk by desk instead of reading as spam. Slack will not put message content in that email if the recipient is on a social domain such as Gmail, or if their address does not match the sender's domain. Copy the rule: same domain as the sender gets the full content and is treated as a colleague, everyone else gets a bare invitation with no content, which is what makes it safe to ship the work in the email at all.

done whenDoes your invite flow behave differently for anna@acme.com than for anna@gmail.com? If the two paths are identical, you cannot safely put real content in the email, and the invite will always read as an ad instead of an internal message.

Remove the second gate so the rest of the company does not depend on your first user. Slack ships a reusable invite link good for up to 400 people for 30 days, and approved email domains, where anyone with an approved address joins the workspace without an invitation. Give your first user one pasteable link plus a domain rule, so person eleven arrives without person one lifting a finger.

done whenCan employee eleven join without employee one doing anything manual? If every new arrival needs a fresh invite from the person who brought them, your growth is capped by one busy person's attention.

Charge for time, never for people. Slack's free plan is free forever with no cap on how many people are in the room, and what runs out is the record: 90 days of message and file history, with the previous 12 months behind the upgrade. Let the team pour its work in at zero cost and put your paywall on something that only becomes painful once that work exists, so nobody ever hesitates before adding a colleague.

done whenDoes adding the twentieth person cost your user money or an approval step? Add up what a user pays, in cash or in friction, to bring in one more colleague. Anything above zero is a tax on your own distribution.