Often no one will join my public game (with a friend or without), but then many players will come in/out. How does Blizzard match/group players into games? Are all games filled to two players first (then three, four), or what are the rules? Is it based off of latency/ping? Does it detect when you're having trouble (read: dying often)?
I've read it first tries to fill up all the single plays so its 2 first , then it fills up the 3's so its 4 players , then it goes back down to 3. Source: DB3 Forums