In our experience, there are three basic ways to handle comped-dinner tip situations.
First, it's an unwritten casino tradition for the "compees" (the guests of the "comper") to spring for the toke. If your guests aren't aware of said tradition, you can invoke it, or you could just say something like, "Dinner's on me. Tip's on you."
Second, however, if you're treating guests to an extremely expensive meal, including heavily marked-up wine or alcohol, the tip can get astronomical in a hurry. In this case, and depending on how you feel about the compees, you might consider being a total sport and seeing to the toke yourself. Make yourself feel better, if necessary, by rationalizing that you're treating friends or family to a nice dinner for 15-20 cents (depending on how George you are) on the dollar. Turn it into an IOU by saying, "Tonight's entirely on me. You can treat next time."
Alternatively, you can offer to split the toke; that way the meal costs all parties 8-10 cents on the dollar.
Finally, you can gamble for the tip. Loser pays. This is a fun way to end the meal, especially if no one at the table will get hurt too badly by having to pony up 15%-20% of a big tab. Bring a deck of cards along and cut the pack; low card loses. Ask the waiter for straws for each diner, then cut one; short straw pays. Ask the waiter to pick a number from 1 to 100; whoever gets closest to the number loses.
The main thing is knowing exactly how you want to play it before you sit down to eat. That way you don't back yourself -- or anyone else -- into an uncomfortable corner at the end of a nice comped dining experience.
