This creates a direct connection between a specific group, on the one hand, and a member having, or wanting, to get a paid membership, on the other. And this is what I mean by the space warping around groups.
Perhaps the gravity of the situation could be mitigated by the wording of the notice. It could say that unfortunately that group has no free slots available, but you can purchase a pass to join any group, including that one, ...
I think that's better than simply telling them "you cannot join because the group is full" and then letting them figure out how to contact someone, or if they should even bother. That's not just a warp, that's a brick wall*. Any response that suggests that they contact the group's owner would create a lot of extra work for the owner, which I I would rather avoid.
Neither member would be aware of how the other got into the group, unless they got into gossiping. Yes, I understand the problem that some members might think that they've got stars upon thars, but the moral of that particular story is that there really isn't a difference.
Nor is it a problem really if everyone decides to buy themselves a star - more money for the guy with the star machine (whether that be the group owner, or Mark, depending on what someone thinks is a star).
*A conceptual relative to a black hole, except it keeps you out instead of keeping you in.