Once you have registered for a forums account, do the following:
1) Click "User Control Panel" at the top of any forum page.
2) Select "Usergroups" tab from that control panel.
3) Select the radio button for "Guild Members" and click "Submit" at the bottom (make sure "Join Selected" is in the drop-down menu).
4) Wait. The OV officers will get a notification that you wish to join the group and will approve you!