Make the core action of your product inherently involve a second person and stamp your link on it, so every ordinary use naturally recruits the next user without paid marketing.
Calendly
Calendly is a scheduling tool founded by Tope Awotona in 2013 that replaces email back-and-forth: you share a personal booking link, and the invitee picks an open slot from your live availability. Awotona bootstrapped it with roughly $200,000 of his own savings, having failed at three prior startups, which meant he could not outspend incumbents on marketing. Both stories below trace the same insight from two angles: distribution was not bolted onto the product, it was designed into the core action, so every meeting booked recruited the next user.
Virality: the invite is the ad
The problem. Awotona had no network, no co-founder, and little capital, so paid acquisition was off the table for a product entering a crowded category. He needed each user to generate new users without a marketing budget or an engineered referral program. The question was whether the act of using the product could itself do the acquiring.
The approach. Calendly made the fundamental unit of value, sending someone your booking link, an inherently two-person act, and stamped every booking page with a "Powered by Calendly" badge. The invitee could not book a meeting without encountering the product in the exact moment it solved their scheduling pain, which prompted them to sign up for their own link. The K-factor lived in the product's reason to exist rather than in a bolted-on referral mechanic.
How it solved it. The invite-based loop drove adoption to more than 10 million monthly users and over 50,000 paying customers by 2021, with roughly 53% U.S. market share in scheduling. Because exposure was a byproduct of usage, CAC trended toward zero, and the pandemic's shift to remote meetings turbocharged the effect as sending a scheduling link became default etiquette.
Growth Loops: the free tier as catalyst
The problem. A viral invite only fires if the sender adopts first, so any friction on the initial signup would break the loop before it started. Awotona also had to convert organic individual usage into durable revenue without a sales force he could not afford. The loop needed both a frictionless on-ramp and a way to compound into money.
The approach. Calendly ran a generous freemium model (a free tier with one calendar and one event type let anyone start instantly with zero purchase decision) so senders could adopt before the loop fired, with paid tiers historically at roughly $8 to $16 per user per month unlocking multiple event types, team scheduling, and integrations. Individual free users then brought Calendly into their companies bottom-up, and team and enterprise plans monetized the org once scheduling became the default. Each new user generated more invitees, who became more senders: a self-propagating loop with the free tier as its catalyst.
How it solved it. The loop let Calendly reach $100M ARR with fewer than 250 employees, extraordinary capital efficiency for a bootstrapped company that raised only about $550K in seed before scaling. In January 2021, OpenView and ICONIQ led a $350M Series B that valued the company at $3 billion on roughly $125M in revenue, about a 24x multiple. Recognizing that a loop built on one simple action invites competition (every calendar vendor now offers scheduling), Calendly later layered a product-led sales motion and enterprise plans on top to convert viral reach into contracts and switching costs.