Keep in mind it is possible to open your game to the public and not have anyone join right away. I've had games open for over an hour before a player dropped in. Maybe they opened the game, played for a while, and then went afk. Then you popped in.
Afaik, you don't get exp sitting in town, as when I play in Co-OP and kill things while friends repair in town, they don't receive the exp, whilst I do.