I'm part of the local LUG (Linux Users Group) and despite our association is not exactly a startup (founded in 2001 if I remember well) we're not organized like we would want to be. Bu we're trying!
In fact, among other things, we move away from "closed source" services like slack (a very powerful chat service, with tons of users around the globe) because our goal is to support Open Source Software.
Entering rocket.chat, an open source alternative to slack. And the good news is: it's great! It has everything we possibly need: mobile apps (android and iOS), desktop apps (linux, OSX and windows), bots, private groups, APIs, integration with other services... and Slack import :)