My DD (18) is bringing her BF (22) with us this Christmas. They'll have their own room as they are already in a long term relationship (nearly 18 months). He spends more time at our house than at his own and although a pain in the **** he is pretty much part of the family. When we get back they are looking for their own place.
I don't know how different things are in the US, but over here as long as the kids are sensible and respect our rules, we probably let them get away with a bit more. They both have good, responsible jobs and are paying their own way so cost isn't an issue.
A few years ago we allowed our DS (16 at the time) to bring a friend from school, we paid for everything from flights to meals and everything in between, his parents contributed nothing and gave him £70 ($110 at the time) spending money. We didn't resent this and considered ourselves to be giving him a once in life-time experience. He still mentions it now 7 years later and apreciates what we did.
As for the original question, would I allow a DD (16) to bring along a BF? It depends on several things; how long have they been together, how well does he get on with the rest of the family, do you trust them both, do you believe that they'll respect your rules? Only you can answer these questions and then you'll be more able to decide. One other point though, if you say no, will DD say that she won't go without him or cause a row 2 days before you go, so that you'll go without her? Devious creatures teenagers, aren't they?