BizSugar.com › Startups › App Virality Hacks: How Trello and Asana Grew to Millions of Users

Trello and Asana grew their user base by using strategies that propagated the apps to teams in organizations.

Here are some of them:
1. Freeware. Allows users to use the apps for free.
2. Work email signup linked work email domain to an Asana account that allowed anyone in the domain to join without having to link the organization to their account.
3. Constant reminder that the app should be used in a team.