It happens often, at least 3 or 4 times a day. It can happen at any time: the first time I log in, during the character selection time, midway through, a match, while I'm browsing through the store. The privacy settings are set to adult defaults, and the option to join multiplayer games is emabled. And no, I never log off during the score screen, I always wait until I'm taken to the main menu and can see how much XP I earned