I don't think any of the mass-market lines allow guests on board, unless it's for some special event like a wedding. I believe some lines have a special program for an on-board wedding pre-departure.

Crystal is a high premium line, and I'm sure they do things very differently. I've sailed on Carnival, NCL, RCCL, Princess, and Celebrity (in increasing preference order). Haven't done any of the premium lines yet. Doing a Uniworld River Cruise in a couple weeks.