The 90 Day Pax Carrying Rule is a legal date limit on activity, just like a valid Rating is - neither say anything about the competence of a pilot on a particular model of aircraft; therefore the abundance, or lack, of Instructors is completely irrelevant.
If someone is competent they can either complete any T/Offs and Landings required solo, before they carry passengers; or as PUT with an Instructor (if they wish).
If someone is not competent then they need an Instructor (and must fly as PUT). They certainly do not need a Passenger who, although they may offer advice, has absolutely no authority to ensure that advice is followed.
In our group the rules are clear. Although there is a 90 day legal rule, we work off a 42 day limit, otherwise you get checked out by an instructor. I doubt many clubs will allow a pilot to rent at the legal limit - they'll have local rules