I can't be sure of it.
Games that have just started with a friend I cannot join, either there is no option to or it says I need to be invited.
Also, I can start games and others tell me they cannot join.
Finally, I was running a multiplayer solo (because nobody joined), while I was waiting on blood to refil to take out the boss, a friend joined me. She said the option to join just showed up. Should it be possible to join a game at the end with one enemy left?
I think a bug or bugs are very possible with joing multiplayer games. Anyone else have these kinds of experiences?
If so please try to relate as much info as possible, if it is a bug it will be found and fixed faster.