Audive
http://audive.co/Audive is the ultimate music collaboration and recording tool for your mobile device. It is all about capturing ideas, beats, or full songs on your own or with anyone, including your friends, on the Audive community.
Audive is useful, it connects people through music, it helps musicians build confidence through collaboration.
Angel list: https://angel.co/company/audive
Member count: 1-10