What is the best way to run a campaign when not all of the players are going to be present regularly? Mainly I'm thinking about characters gaining experience, should we automatically give all players the same experience rewards to keep the game balanced, or allow them to level up their character only when they are present to play? The reason I'm asking is that our RPG group is running Descent as an alternate game for the nights that one of our members are unable to make it out, so we will always be at least one man down, but it probably won't be the same man missing every night.

Thanks

Play the game with the same number of heroes every time, and just rotate who plays which heroes as necessary. All heroes are present to participate in each quest, so they all advance equally.