Normally you would click on the User Control Panel, then click on the tab that says Usergroups. Click the select button next to the forum name you want to join, then at the bottom select the option to Join the Group.
Here is the direct link: ucp.php?i=167
The radio box is on the right side, you would check that box, then click on the Join Group submit button at the bottom.
Don't worry about doing this, though, as I've added you to the group. If you no longer wish to view that forum just follow the same thing, but instead choose the Resign option.
Hope that helps.