I've seen this question asked before, and in-game to. This thread will (hopefully) be dedicated to showing opportunities that will help anyone can get involved.
Let's start small.
Play the game! The more people we have online, the better, and more fun the game will be.
Join IRC. Answering questions, being there, and more than just "idling" IRC, are all ways that make the community look positive, outreaching and well-organized.
Suggest, debate, or vote for a feature. The more input and feedback we have, the better the ideas turn out.
Beta test maps. Too often I've seen good maps that have received feedback like "Oh awesome! Screenshots look great" which is a fine response, but I love when I see some of our veteran users post more in-depth responses. Don't be afraid to post CONSTRUCTIVE-criticism because someone might take it personally and not like you. I like to think that if we set a positive enforcement (what word am I thinking of?), that won't happen. Discuss ideas and features to improve the map, even if you feel you might be ridiculed... having the courage to offer help, even to someone who may be to immature to take it, is a bold trait I'd like to see in more of our members.
Take a picture. Huh? Take a picture? I can help by doing that? YES! Myself and several others could easily take any good picture you take, and transform it into a high-resolution texture, that will be included in several maps! If you're interested in helping here, please see the Community Texture Pack.
Suggest an idea for a "model", or 3-D object, to be included in the game. We have many experienced and inexperienced modelers who are willing to make YOUR ideas in to object that will be in many maps. All you have to do is SUGGEST an idea for a model! This is as simple as "Umm, I'd like to see a 2-liter of soda..."
Reply to posts in the Help and Support board. Even if you don't know how to help someone, run a quick search and see if you can't find the answer to their problem in another post... 90% of the time you can. If not, ask for specifics and other stuff that will help others to help them. Just QUICKLY replying to random people who wander on to the board, gives them the sense they aren't being ignored, and makes them feel like the board actually has real, human users. Putting a human face on this game is a big part of success in growing.
Record sounds. You may be surprised to know that Paintball II is in need of sounds. A list is posted in the wiki, so grab your microphones, and start helping!
Update our Wiki. Add stuff to it, and prepare articles. Soon we're switching to MediaWiki, and when that happens, we'll need people to port the old wiki articles over, as well as write new ones.
Advertise. Have room for a banner on your site? Are you a member of a forum that would be interested in Paintball II? Just tell someone about and you've already helped Paintball II. The more friends you lore in to Paintball II, the more friends you have here!
Advanced users
Make a map! Mapping is easy to do, hard to do well. It takes practice. Learning to map, and producing QUALITY maps for Paintball II would be a huge service. Get started by read the first few posts in the "Mapping" board.
Port Paintball II! Want Paintball II on a Mac? Make it. We have the source code on the "Files" page, if you can do it, ermm... do it?