Crashing private parties may be fun, but it definitely is not a socially acceptable thing, unless you live on a college campus. If you want to attend a private party the right way, there are some acceptable ways to do so. You can be present and participating in the private celebrations in all of these ways.
There are only a couple of ways to attend a stranger's party with the host's permission; one of which is working in food service. If you work for a caterer, you can attend lots of parties, feed everyone there, and mingle at the same time. Usually, the host or hostess will allow you some party time once your duties are complete. You may also be serving the entire duration of the party, in which case the vast majority of the afternoon or evening you will be interacting with a lot of guests, listening to music, dancing a little bit (when no one is watching) and eating some of the leftovers. In short, you are crashing the party in a socially acceptable way and getting paid to do so.
Working private parties as a DJ is another way to attend a party with the host's permission and get paid for it. The host hires you play music for several hours and do your DJ thing, and you get to dance and party with the rest of the crowd. If you are also allowed to take requests, you can interact and socialize with the group. In fact, you probably spend even more time partying than the food service and catering crews do because you are the entertainment.
Everything from strippers to famous singing impersonators to drag queens all count as live entertainment. If you work for a live entertainment company, you get to attend several private parties with the host's permission and get paid for it. You often get breaks built into the entertainment sets you do for each party, which gives you even more time than a DJ to mingle, drink, eat, dance and have fun. The best part about being a live entertainer for hire is that you get to stay all night, whereas the food service/catering people often pack up and leave when everyone has been fed and the DJ only plays for a specific block of time (although some DJ's can be booked for the entire event).