When I first started going to Sundance, it was just a bunch of filmmakers and a bunch of filmlovers. Filmmakers had no entourage. No one told them what to do or what they thought was right; instead they shared information and secrets. But that was then.
For the last ten years, it has seemed that filmmakers arrived at major festivals with a horde in tow: lawyers, agents, managers, producer's reps, foreign sales agents, and publicists. The list actually goes on from there. But that was then.
These days, recognizing that a sale is very unlikely, how much do you really need? There's definitely another few posts worth of material in that question, but I can tell you that the one I think is critical is the publicist. After all, it is all about getting the word out about your film.
The traditional media still holds the most weight (okay, that's debatable), and any a publicist worth their salt will know how to reach them. More importantly, the publicist will know what these critics and journalists look like, and will be able to find out what they thought of the film immediately. Their opinion matters as it influences everyone: buyers, festival programmers, independent bookers, and other journalists.
The publicists also know the distributors and as long as you want to keep Plan D (sell your film) alive, that is invaluable as the publicists can help facilitate meetings with the buyers.
A publicist will help you draft your press notes in advance of the festival and arrange key interviews. Sometimes they can even help find a corporate sponsor for a party (more on that later). The publicist will collect all of the press you receive, and survey the journalists on their response. They will collect all this material so you can share it with everyone you reach out to later.
How do you find your publicist? Well these days they often find you if you get into Sundance or a major festival. The key filmmaking community organizations like IFP and Film Independent can also help direct you. Maybe I can put together a list and post it here (I will get back to you on that).