I've done plenty of threesomes, a couple of foursomes, and a fivesome. I even lived in a MMF triple for a few months, with 2 bi men. THAT was a lot of fun!

Groups can be fun as long as everybody is on the same wavelength. Be sure that everyone wants to be involved, not just going along because they believe their partner wants them to. That will spoil it for everyone. Be sure that everybody agrees on whether there will be any same-sex involvement. It can be quite uncomfortable for everybody if there are misunderstandings in that area! Also, be sure you all understand what it is about - is it for fun, is it to deepen the friendships/love that are there, is it about "partner swapping", or whatever else.

If someone is not secure in their relationship, this can lead to various emotional or apparent emotional bonds being formed, and long term couples breaking up and someone ending up with someone else... at least for awhile.

Above all, be certain you are being biologically safe! Don't leave with something you did not anticipate having!