Actifit is always my go to app when I'm talking to outsiders. People are already familiar with Fitbit and pedometers, actifit has a nice interface and it's relatively easy to help people start posting.
The tribes help a lot, too - now I can show people a dedicated feed of posts that will interest them and now good curation is starting to kick in and the focus of feeds is improving.
We're getting increasing numbers of ways to help people easily sign up for an account and get doing whatever it is they are interested in.