The Avengers are a little different, because they're self-funded - Tony Stark owns Avengers Mansion, and pays for its upkeep, as well as the technology used by the team, and, presumably, their salaries, too. (At least, this used to be the case - now that there's at least two different Avengers teams and last I checked the mansion was wrecked, I don't know what's going on with them.) Here, on the other hand, Moon Knight is essentially hiring Power Man as an employee. He's a member of the team as long as he's paid well.