If you've made it this far in the guide (and haven't just skipped to the end - that's cheating, you scoundrel!), you should know most of what there is to know about the game. However, remember - the game is in beta, and you're not just a player, but a tester. You can help the game evolve and become even better! Here are some things you can do:
Make an account on the forums. The forums are where all serious discussion about the game occurs, and you should definitely browse it, and if you can, contribute to it. This is where people propose ideas, report bugs, and discuss changes to the game. Going on IRC and chatting in-game help too, but neither is going to have as much impact as posting on the forum.
Have an opinion? Tell it to the dev team! Nobody's going to know what you think is great and what you think sucks unless you report it. In particular, when things change, ask yourself what you like and dislike about it, and report it on the build thread!
Report bugs and exploits. When reporting bugs, try to be specific in describing the situation that led to the bug, since it helps the debugging process. If you can reliably reproduce a bug, include the steps in your report. Videos and screenshots help a lot too. And this isn't just limited to bugs - if you see an unintended feature in a game (e.g. something that allows the system to be gamed), it's worth reporting, too. When hunting for bugs, it's good to have a bit of a "breaker mentality" - think about how things can go wrong and test them. If using hurricanes makes you lag, for instance, get a few hurricanes together in a team and see if you can crash Flash! However, please don't hack the client - security is not a feature of the game yet and therefore shouldn't be tested, and if Rob and Co. get wind of hacking activity, you can be sure they'll pull out the banhammer pretty quickly.
Invite new testers. You can get a list of invite codes by entering /codes into the chat (if you don't have any, don't worry - you'll normally get them when a build is pushed). Don't give out codes indiscriminately - try to ascertain whether a candidate would make a constructive tester before giving them a code. People who rudely and incessantly beg for codes tend to make terrible testers.
Last, but not least, this is a game, so:
Have fun!