The first thing that comes to my basically paranoid mind is you'all write a tune, it gets picked up as a theme song for a major network television sit-com (it happens), and along comes Jones claiming he was the actual author and demanding royalties. Wouldn't be the first time that happened. What's to prevent this guy from copyrighting anything he hears at your rehearsal? I'm just saying. Unless you have a written agreement with this person, including him in song writing sessions just seems fraught with potential problems. Not the brightest thing in the world to do, in my opinion.
Technically, a copyright for a creative work is created when the work itself is created. When you "copyright" something, you are actually only registering the copyright. This makes it easier (i.e., possible) to enforce your copyright. It also makes it possible for people to claim rights to things they don't deserve.
There are certain other peculiarities to the copyright laws. You can't copyright a song title, for example. I found out about this back in the day when I was riding in an elevator and heard one of my submissions come over the piped in music. The tune was re-written some, but it was my tune, no doubt. This kind of thing was not unusual back then and when it happened, it could actually give you a little more standing with producers, etc. It was usually best not to make too big a deal about it. Producers were looking for as sure fire a bet as they could get, so any proof your material was commercially viable could actually be helpful in that particular club.
It's kind of a shock though when you see a pretty big chunk of a script you copyrighted produced almost verbatim on the tube! I hadn't even submitted the dang thing! So know that copyrights are a rich man's game. You have to enforce your copyright. Not always easy, and always expensive. Another interesting thing about copyrights is that if you don't enforce your copyright, you loose the rights to the material. Lot's of guys lost the rights to their material because it was demonstrated that they didn't enforce their copyright.
I personally wouldn't permit anybody to participate in a song writing session I had anything to do with unless an agreement had been signed waiving their rights to the material, or defining their contribution and what arrangements are contemplated for remuneration in the event of commercial development of the material. Then again, I tend to be paranoid. Good luck in the jungle.