Not much appears to be available on this topic, but according to one GameFAQ post:
Even though you don't get a pay raise for grouping with someone weaker, the lower ranked player will get pay raises and promotions. If you keep playing then sooner or later they'll rank up to your level and you can start earning pay raises again. You'll still earn a ton of points since the matches will be easier (for you anyways assuming your rank is higher) and you still reap the benefits of a high pay grade.
To me, this sounds like you are stuck playing on the lowest pay grade between the players. Once the lowest person's pay grade meets or exceeds the next highest, the next highest person will begin to rank up. This continues until everyone is evenly ranked, to which everyone will be able to continue to rank up at this point.
Part of this conflicts with another GameFAQ post however, which states you play on the highest pay grade instead (the rest is the same):
Almost, except it seems to me like the difficulty is set by the highest rank player, while only lower rank players can go up. Which I find... weird, but whatever, that seems to be how it worked.
More people seem to agree that it's the lower difficulty that's taken into account first (based off the length of the two posts). In my opinion, that makes more sense as well.