Get some more details!
I've been in this position before, and you MUST know more about what this film will be used for before you make a decision.
In my case, we had a local news program want to tape one of our Vampire LARP sessions for a "special halloween story". It sounded like a great idea to us at first, but after asking more questions, and seeing a tape of the previous year's "special halloween story" it became apparent that these people were really just looking for some "wacky satanists" to film. Their previous story completely distorted facts and was nothing but over-blown sensationalism that made the people involved look like freaks. We thanked them for their interest, but declined to be filmed. (Then they had the gall to park a van outside our venue and attempt to film people coming and going that night!)
Anyway, were I in your position, I would ask the following:
Exactly what is the topic of this documentary? Can you see some other clips from it? Can you talk to any of the oter people that are appearing in the film?
What else has this film-maker shot in the past? Ie. are you signing up with Ken Burns, or Michael Moore? They both make documentaries, but there's a BIG difference between the two!
When and to who will it be shown? Is this a "Wacky Gamers" kind of film or something like "Uber Goober"?
Do you get any creative input/control? Can you see a copy of the footage before the film is complete?
Get some more details before you make a decision!