Sadly, Resident Evil Revelations lacks any possibility for the player to communicate verbally with randoms online.
However, there is still a way for you to express your decency to your partner in online Raid Mode - Elevator Knifing!
Whenever the both of you are waiting in an elevator together, start melee-attacking your partner (this does not actually hurt him/her). If your partner is a good sport, he or she will start melee-attacking you back. This will persist until the elevator arrives, at which point it is back to action.
I say we establish this as a universally-accpeted ceremony.
So, always melee your partner when in an elevator. If we do it enough, it should start to spread naturally.
Have an excellent day.