Unless it is one of those groups that sits and drinks coffee and takes up a booth and orders no food. (totally guilty of that as a pre-21 year old kid)
We would leave a buck a person and even that didnt seem like enough given the real estate we took up.
It all depends where you are at I guess. Nice restaurant, 20% is great. Bill tops 100, they get a great tip.
At a diner, where prices are cheap and turnover is the game, you gotta at least drop the 5 for rent if you are clogging up their seating area.
If this were a true story we were responding to, my question would be "how long do you stay?"
the tip may be fine, but not so much if you loiter and keep them from getting other customers in their area.