Just seen this thread and I'm going back to Genghis' original Q by relating my experience of a very similar situation.
I was in exactly the same situation about twenty years ago. As a member of a flyng group with about a thousand hours and as the only professional pilot in the group, I was the person who did the group's acceptance check rides and type 'training'. The group operated from an airstrip which it has sole leasing rights to and operated a slightly unusual type with a stick and throttle operated by the left hand which prospective group members had usually not flown before, so a fairly high standard was required. Also, we required 100 hrs P1 since the award of the PPL. This seems quite high, but as the group members/aircraft owners were going to fly unsupervised, it was quite desirable plus also an insurance requirement in this group.
When I accepted the job of doing the flying, I laid down my own terms, in that if I wasn't happy about any of the prospective group members then they weren't in - and this was accepted by the rest of the group. And it all went well for about two years. Typically, the prospective member flew the aricraft with me for a looksee and if they liked the group, they would join provisionally and once checked out on the aircraft to my liking, paid the remainder of the share and were in. The insurance company were happy with this situation, and as for logging the hours, I would log P1 from the RHS, as for the candidate, it was up to him. But one day, we were faced with a big maintenance bill and there was some pressure to accept two new members to the group. I was not happy with either. One was underconfident and not a particularly profficient pilot, but eventually, he convinced me he was safe and I recommended to the chairman that he be accepted. The second was overconfident and irresponsible as well as not being a particularly good pilot. I told the group commitee that we had our first rejectee, but I was over-ruled due to him already having stumped up the cash and also the fact he was a close friend of the Chairman.
This chap subsequently gave me lots of missed heatbeats by flying in unsuitable weather, low flying and dubious decisions and eventually proved me right by crashing the aircraft - without injury to either himself or his daughter, which considering he managed to remove both wings in the crash was a miracle!
So my advice is to stand your ground and make it clear to the rest of the syndicate that if you aren't happy, then they aren't to fly the aircraft.