I haven't been able to find a way to join a site without going to a question and then trying to comment. At that point it asks if I would like to join the site. I'd there a way to join a site in the Android app without having to try to comment on the question?
There is currently no way to explicitly join a site on the Android application. Any interaction you try to do with the site: Asking a question, commenting, answering, voting, so on, will allow you to join the site before fulfilling the action.