I should create message extension app for MS TEAMS. This app should know the roster of a group. For that, the app should be joined the roster. It means the user should add the app to the group chat. Is it possible to do it without the user? Somehow dynamically or something?
You can add an App automatically to a channel inside a Team, using the Microsoft Graph, but I don't see an option to do that on a group chat. However, on the beta endpoint you can get the group chat member directly, without needing to add the app initially. See here for that.
It won't help you if you need the app installed for the message extension capabilities, but at least it will give the roster, which is what your question is mostly referring to.