Agree with alex90. It may just be something simple as an insurance or group policy that states that everyone flying as PIC (which your instructor would be when he's teaching you) needs to have a familiarisation flight ("check out") on the aircraft. Which is, generally speaking, a good policy to have. And it's a good and decent thing that this flight can be done with anyone of the members, not just with the designated group instructor. (*)
If your instructor is anywhere near as experienced as you've led me to believe, that flight can probably be done in 15 minutes. And in the future gives other members the additional benefit of an extra instructor that's available to teach on the group aircraft.
What I don't get, though, is that you are "part of a group" that apparently owns a VP/CS aircraft, and that you have not yet been endorsed with VP/CS training. IMHO as soon as you join such a group, it would be the very first thing to arrange, as without that endorsement you can't legally fly the aircraft. From that I conclude that you're a new member. In which case I would advise you not to start kicking up dust just yet. Talk things over with the group members. See what they think of an unfamiliar instructor teaching on their aircraft, and what they think needs to be done to get this instructor on board. As said, they may benefit from it as well.
(*) From a legal point of view the situation where a non-instructor, or even worse a PPL, checks out someone else on an aircraft, is very murky. Who is the PIC in that case? If the "student" is the PIC, what is the responsibility/liability of the "instructor" if something goes wrong? And if the "instructor" is the PIC, then the "instructor" - assuming he only has a PPL - needs to pay part of the costs and the "student" can't log the flight. Plenty of discussions on this elsewhere on this forum, no need to rehash them here, but to preempt the discussion a lot of groups/clubs specify that these check-out flights can only be done by qualified instructors.