I think most airlines allow non-elites to check-in with elites - airlines that I've personally checked-in a non-elite - AC, UA, LH - airlines on which my Dad has checked me in with him - DL, AC.

I don't know whether it's an official policy or not - but I can't see any airline forcing spouses/partners/colleagues/siblings to separate during check-in.

Also, generally speaking, you can bring at least 1 (non-elite) companion into the lounge with you - but that's not always true especially when accessing a partner airline's lounge.