Although X used to let passengers join CC on board their first cruise, my roommate on last summer's Constellation cruise was told she had to wait till after the cruise and join online.
I have always been able to bring a non-CC roommate to CC functions. Tomorrow 4 of us are leaving for SJ for a S. Carib. cruise on Summit. Whether I will be able to bring everyone remains to be seen. My own roommate has cruised X once, but didn't join CC, so maybe they'll let her join on board. I'll let you know.