The situation described does not seem to require an instructor rating or any specialist examiner qualification. The OP is charged by the syndicate with merely assuring himself that the wannabe is unlikely to bend their aeroplane, damage its engine etc or bring discredit on its owners (through poor airmanship, overloading or whatever).
Perhaps the syndicate needs to publish a simple set of requirements e.g. use this particular checklist, bring your own headset, charts and if not flying regularly carry a safety pilot. Set up a simple limitations and loading exam for them to complete prior to flight and don't fly until it has been done and graded.
In the situation described, I would then brief along the lines that, while technically (for legal and insurance reasons) the PIC for this flight I would be acting as safety pilot only, would not contribute in any way to the flight but would at the end of it decide whether or not the candidate needed dual with a qualified instructor, or another familiarisation trip as 'co-pilot' with one of the other syndicate pilot before going solo, or would be cleared to go.
If at any stage of the flight I had to intervene or take control, it would automatically require the dual or safety pilot limitation.
Only someone totally thick would fail to understand that messsage, and syndicates don't need these people.