Imagine some sort of companion school or something, idk someplace where they would all meet up to trade info and tricks and stuff.
Barbara would teach them how to dress/behave/etc in certain historical periods, and what not to do. Ace would teach people how to make their own explosives (because that might come in handy some day). Martha, Grace, and Harry could give everyone some basic first aid training. Zoe could teach them some basic computer skills (especially pre-21st century people and Clara).
The UNIT guys could give them a lesson in properly holding and firing a gun, because even if the Doctor doesn’t approve, sometimes you need to take out a Dalek or two. Sarah and Jo teaching them how to break themselves into and out of places. Romana showing everyone how to at least materialize and dematerialize the TARDIS (because it’s really not that difficult).
Rose teaching them some basic gymnastic skills that could come in handy in physical situations. Peri showing them all she’s learned about various plants on different planets that they might end up on (and teaching the alien companions about how to recognize dangerous Earth plants). The human companions giving the alien companions tips on how to blend in on Earth, because let’s face it, they’re going to end up there at some point.
Everyone sharing a drink and talking about the most ridiculous way they’ve taken out a Dalek, or the weirdest situation that the Doctor got them into.
Just…companions helping companions and being awesome in their own way.