The only problem I have with this game is that the push notifications kept making my fucking alarm go off, so I had to uninstall it. The game is fun, but that issue is annoying as shit :/. Ninja Royale has that problem, too.
Well in D2 it was based on wherever the host was in the storyline, so if they were in Act 4 and you were just starting Act 1, you wouldn't be able advance because they had already finished the quests. I assume it's the same in D3.