If you had a website which said:
"I am going to France on Tuesday at 10am, anyone want to come and we'll share the costs"
Then I think that would be ok, and the flight is intended anyway.
If you said.
"Who wants to go to France next week, time and date to suit, we'll share the cost"
Then that is not ok.
There is a website in the USA called "Pilot Share The Ride" where pilots can advertise flights and I think passengers can advertise that they want to go somewhere, and I presume that is legal.