DL extends elite baggage allowances to passengers traveling on the the same flights and whose reservations are linked to those of the elite, up to a maximum of 8 companions. Note that you will have to check in bags with a DL agent for this waiver (don't pay the baggage fees online.) This is official DL policy and has worked for me in multiple airports, particularly LAS and BHM, but the TAs in SEA did not know of the rule. As several FT'ers learned at the SEA Thank You DO, if DL ever tries to charge you a baggage fee higher than you are supposed to pay, pay the fee and call/email DL who will send you a check for the extra amount.

AS does not extend elite baggage allowances to passengers traveling on linked reservations.

As for preferred seat assignments, AS seems all over the map on how many companions on linked reservations can have preferred seat assignments. DL extends preferred seat assignments to passengers on linked reservations, but you will have to call DL. Economy Comfort seating for elite companions should also be discounted for international flights, but YMMV.

Except for Economy Comfort seating and upgrades, the general rule when flying DL is that MVP=FO(Silver), MVPG=GM, and MVPG75K=GM/PM.