I run a session for 4 static players, and a friend of ours recently had some time open up. I know he's interested in RPG's and would probably enjoy playing. That being said, I don't want to have to add a 5th static for scheduling reasons, and his schedule still is more hectic than the rest of ours(plus we've had some scheduling trouble recently).

I'm wondering if it would be fun to have him show up for a session or two here and there and play more of a boss like character who's an antagonist to the group.

Has anyone ever tried something like this? Are there any issues I'm not considering. This is my first time running an RPG in general so I'm pretty inexperienced.