Just send out a quick question when you first sign in, I usually use "room for another?" this just lets people know your looking to play, often if one of their party members quit they will send you back an invite or often there is a party that is short a member. Otherwise when you sign in the message pops on the screen usually when people are in the middle of a game by the end they often forget you signed in or two or three others signed in afterwards so you get lost in the suffle.