I think the majority of events held don't mind spectators, but sometimes the format of the game or area of operation won't allow for it. Always get permission first from the game organizer before making the trek out.
As for taking pictures and/or video, it's the same thing, get permission first. While most of us don't mind posing for the camera there are a few in the community that would rather not have their image plastered all over the internet (each has their own reasons).
Don't be afraid to ask questions at the field during downtime (before and after games). With most airsofters, once you get them going on the topic it's hard to get them to shut up.
Once last note, be mindful of asking permission before handling anyone's weaponry. Simply picking up one without asking is a no-no. Most won't have issue, but again there are some that have put so much money and effort into their builds that it's treated like a newborn and they can be very protective of it.
Hope some of this helps as far as etiquette is concerned. Welcome to the drug...er, I mean, sport known as airsoft.