As for location, you have two options:
1) All adventures start and end at the same spot, so it's easy to bring different PCs into the next adventure.
2) Adventures might start and stop at different places, but you need to have ways to get different PCs to that place if they're in the next adventure.
#2 can work, but it has to be believable. If you start in one town and end in another, you can easily come up with an excuse for the other PCs to have traveled their on their own. It's a little harder if the adventure starts in a town and ends up in the Abyss, or something.
"Hey Fred! Imagine seeing you here!"
"Yeah, I know! The Abyss, right? How about I tag along? Hey, where'd Joe go?"
"Joe? I don't know. He had some errands to run, or something. You know. In The Abyss."