1) Find a group that you want to join
2) Make sure you are signed in with your .net passport
3) Click on, "Join this Group," in the top left of the screen. See [url=http://img23.exs.cx/img23/7355/JoinGroup.jpg]here[/url] (circled in red)
Thats it! You may not become a member right away depending on whether or not automatic joining is enabled or not. Sometimes you may have to wait for approval from founder.