The FAA is looking for two things when considering the question of whether a
private pilot was carrying a passenger(s) legitimately. One is evidence
that the passenger is incidentally aboard on a flight that was going to take
place anyway. Second is that the pilot and passenger have a shared interest
in the objective of the flight.

In the case of co-ownership, such as in a partnership or flying club (with
stock), would there not be a presumption of shared interest?

For example:

Strictly speaking, if a friend not involved with your aircraft said. "I need
to go to Podunk on Saturday, how about flying me up there?", the flight
would be questionable if you had no prior intent or independent reason to
fly there.

However, if a co-owner said, "I need to go to Podunk on Saturday and I can't
fly PIC until I finish this medication, how about flying me down?, I would
think that your co-responsibility for the aircraft management and
maintenance and similar factors would make this OK.

If you co-owner said, "I need to be on Podunk on Saturday and my wife would
like to meet me on Saturday, how about flying her up and we'll have lunch?",
I would think that would be OK even though it would be questionable in the
case of a non-co-owner.

Anyone care to predict what the FAA would (or should) say? Assume costs
shared properly according to seat occupancy.
--
Roger Long