The simplest solution would be just to add all companions to your “party” when you’re at camp. We already know the UI can handle it.
You could even have a quick screen when you leave camp which lets you change who’s in your team (like Mass Effect or Dragon Age). That would encourage people to mix things up a bit and think about the best team for each mission
+1 ! Always thought the DA system was really good at this. Never understood why other crpgs didn't pick up on it! D: