I'm having a party and I want to add flavors to my already-brewed beer. How can I do that and have it mix?
Any time you mix things into beer you'll lose some carbonation. Any powder would nucleate bubbles very quickly, so the flavoring should be in liquid form. The liquid should be as similar to beer possible: cold, equally carbonated, and not too much alcohol if it can be avoided. Stir gently in a cold glass and serve right away.
Or, if you are brewing the beer yourself just add it at kegging or bottling time.
There are products called Randalizers that can allow you to infuse flavors into beer as it's served. Provided that the beer is 1) on draft, and 2) the flavoring is a solid (hops, peppercorns, citrus peel, etc). An expensive option is the Blichman Hoprocket. Another option I've seen, but I'm not sure how to construct it, was an inline water filter that had been modified. That modified filter also required significantly more CO2 to maintain pressure, I don't think that was true of the purpose built devices.
These work well for fresh flavors, but would serve poorly if you were looking for anything else. I've had beers run through a couple different combinations of herbs, fruit, veggies, and hops, and it was always interesting and exciting. Even if it wasn't always that good.