There are a great many roles we need to fill for this event. Below are some of the Hobbits and other folk known to have attended the birthday party. Look around and see if one suits you. We are particularly interested in filling the roles of Lotho & Lobelia Sackville-Baggins and Odo Proudfoot. If you wish, come as a hobbit we have not had the pleasure of meeting yet.
Yes, yes, of course we will need hobbits ~ as many as we can get, especially for the Birthday Party, and we will need heads of every family so that they can cheer if they are mentioned in a certain speech, but remember, you don't have to be a hobbit the entire event. See more below...
It is not unreasonable to think that there might have been some Elves about, for Bilbo and Frodo visited them often, and they were seen in the woods and on the borders of the Shire. An Elf outfit would be most appropriate for the Hall of Fire event as well.
In the book (and not in the movie), there were a great many dwarves at the party, before the party and after the party. Bilbo himself left with a band of dwarves after he (finally, but of his own accord) let go of the One Ring, and dwarves were seen on the roads of the shire from time to time as well.
It is mentioned that there are woodsmen (and women, of course) that inhabit the surrounding countryside, and not far off is the village of Bree and Archet in which men dwell. Hobbits all knew of the 'big people', and Tolkien notes how hobbits could hide well from this blundering and loud race.
Few hobbits knew of the protection the Rangers provided for them. Much like the movie and books, if you come as a ranger, your may be called upon to keep the peace at the event, and watch after the unwary, for the dark lord has heard at last of the Shire, and he even has a name to think about...
Although there were technically only five wizards in Middle-Earth at the time of the party, perhaps they trained others in their craft., and who is to know that Saruman, Radagast, or the blue wizards Alatar or Pallando might drop by for a chat or a bit of pipe-weed.
Of course, there are many other races populating Middle-Earth ~ some good and some under the sway of the dark lord. And there are those who serve no master at all, but this by no means they are to be trusted while other beings may provide help when unlooked for.