Please remember that all members signed up for raids have priority for invites.  If you did not sign up for a raid, do not complain if you get passed over for an invite.  The only exception to this rule is overfilling of specified slots (this generally only applies to tanks and healers), since this will lead to an epic phail almost always.

Also, please sign up for any raid that you would normally attend, just because a raid is full, does not mean that there might not be dropouts.  Those that are on standby will get first priority invites to replace missing signups.

If this is your first time using this for signup, all you have to do is register yourself, then after you have logged in, select the "Characters" option under the "My Account" menu and add all the toons that you use in the game.  Once you have done that, you will be able to sign up for raids.