How this growth loop worked
One person on a team started using Slack alone, then hit the obvious wall: chat is useless by yourself. So they invited their channel, or their whole team. Those new users adopted it the same way, noticed their own collaborators were missing, and invited the next group. Each cohort of joiners became the source of the next round of invitations, so new active users fed straight back into the loop's input.
Inviting wasn't a tactic bolted onto the product; it was the product. A messaging tool has no value until your coworkers are inside it, so every engaged user had a concrete reason to pull others in. A K-factor below 1 won't sustain growth on its own. It works as an amplifier: each acquired user drags a few extra signups in behind them, so paid and organic channels cost less per net new user.
If you want a loop like this, don't bolt on sharing and hope people use it. Build the product so its core value depends on other people being present; then the invite is part of using it, not a favor to you. Measure the real cycle: how long from signup to the next invite, and what share of users invite anyone at all. A shorter cycle and higher invite rate matter more than chasing a K above 1.