You could actually use either. "is anyone sitting here?" or "is someone sitting here" are both correct. OR you could say "is there anyone sitting here?" or "is there someone sitting here?" all of these are correct and have the same meaning :)

Although there is more than one meaning for each of the words "sit" and "set", in the sense that you are using "set," the difference is that a person sits in a chair and a hen sets when she is laying an egg.

Neither is correct. "setting here" doesn't make sense

correct is someone.