Our company was generous enough to include private aircraft when paying IRS mileage for business trips by POV, so covering approved expenses for a 4-person flight was never a problem. I was always happy to “eat” the unapproved expense as payment for the joy of some cheap time in the air.
I do agree the gray area of affirmatively seeking out passengers to a common destination is hard to deal with. We have done a number of shared expense flights such as to resorts in Baja over the years and all were legitimate “common purpose/interest” where everyone was socially related and stayed (partied?) at the same spot. While all were arranged through informal social contact, would they have been any worse had we arranged them via a “I’m going” post to co-workers on the office bulletin board? Or at a small airport where virtually everyone knows each other?