I played in a game in which the GM had an excellent approach to this that I have since stolen. The concept was introduced in a superhero RPG into which it fit perfectly. The idea is that when a player is away, his character is there, just "off frame." Imagining the game as a comic book, the character in question is with the party but outside the frame in which events are depicted. In all likelihood, he's fighting other villains who are also off-frame or doing something equally important.
In this off-frame state, the PC cannot take any actions that pertain to the main action, nor can he be damamged or collect experience. I have found this to be by far the cleanest solution to the problem of absent players; and it does less damage to the story than any other option.
As for questions of quorum, I generally just want a simple majority ie. 2/3, 3/4, 3/5, 4/6, 4/7.