Well, so you want to help the Pcsx2 project. This is great, we are always looking out for more help. After all, this project is the result of strictly volunteer work. If you want to help us, read below.

You want to help us as a developer
If you have knowledge about coding and you want to help, great. It's the best way you can help us really. The recommended form of communications is to drop in on the #pcsx2 irc channel and look for a team member. From there, with a little patience, you will be able to talk to the devs about your ideas, and good things will hopefully follow

Note that while the emulator is very big and complex, you need not be a guru and/or understand it all to be able to help us. Even if you want to, say, rewrite the GUI, or the memcard manager, or write a frontend for saving the preferences or anything else, we will still appreciate it if it is feasible.

If you want to know anything else, or need help to contact us or whatever, feel free to send me a private message here in the forums.
PCSX2 is and will always be a fun project, you'll find it highly rewarding

You want to help us testing games and reporting bugs
This is also very appreciated. We are always in need of more testing because we obviously do not have all the Playstation 2 games, nor the time to test them all in-depth. The forum "Bug reporting" is there just for that. Enter it, read the topic with the usage instructions, and start to post.

If you own a considerable collection of Playstation 2 games, and feel very motivated about testing, we may pick you for testing our private builds... Contact me if you want to know more.
Note that this kind of testing is not a pleasurable occupation. It's something for very motivated people. Also note that you won't get to have "faster, better, l33t3r" builds. You will have to test horribly broken stuff, over and over again. If you feel like you just want to help by testing occasionally, simply use the forum as written above, it's already very appreciated.

NOTE: being able to compile your own builds of pcsx2 is a serious plus...

You want to help us by providing support in the forums
Well, now that we have a forum we need to give support. If you can give answers to the new users in a helpful way, that will be much appreciated. Less time spent on support = more time spent on coding and testing

You want to help us in another way not mentioned here
Well if you want to help in another way, contact me via private message and we will talk about it

Updated