Meetup's viral loop

Meetup is an online community that takes helps people take their relationships offline. As soon as a user RSVP's to an event, they're hit with this modal:

Why this is really good UX:

• Going to an event is an inherently social thing. After committing to go yourself, you're highly likely to ask friends to go with. This brings more people to the platform, which is a win for Meetup
• The modal text is simple. It makes inviting friends out to be an obvious next step, but it's easy to dismiss if that's not your thing.