These raids will normally take part on Fridays or Saturdays between 12-3pm.
To join any of these raids you will of had to sign up for them via the forum at least 1 hour before the raid is going to start. After the raid has started anyone which has turned up late will not be invited.
Only characters in the guild will be allowed to attend these raids, and you have to say which character you wish to come on when you sign up.