From stories I have read it's 30 min before opening- you just wait with everyone else out front. They usually pick a family of 4 but not always. They also pick families with older kids- no toddlers or infants.
Again, from what I read, the best way to get picked is to all be smiling and having fun, dress "fun", and look like you are just open to some pixie dust! You may see a CM coming around chatting up folks so definitely be talkative.
Good luck!