These features were cut for time.
When faced with getting it out at all versus adding additional features we biased towards shipping. We are investigating what it would take to allow larger party sizes, customs, as well as adding challenges, mastery, and other things from our several miles long todo list.
Not likely for the first run, but something we will strongly consider for future runs of the mode. Obviously if we were to make it permanent we would want to support social play here. We have friends too!!!! (I mean I don’t but I assume other rioters do.)