DW and I were there this past Sunday morning. The sign said the meet and greets were to start at 10:00am, and we got there about 15-20 minutes early. There were no lines, no one waiting and no castmembers around. The doors to the tent were unlocked, and there was nothing indicating that we should wait outside. So we went inside the tent and waited in line in the queue for Minnie and Daisy. At about 2 minutes to 10:00, a couple of castmembers came in and were surprised to see us inside. They thought we had snuck in some how and told us we were supposed to wait outside in line with everyone else and we would be led in by castmembers and the the characters when the doors were unlocked. We explained that the doors were unlocked, and no castmembers around when we had arrived 15 minutes earlier. They told us we would have to leave through the exit and get at the back of the line. We protested slightly and were able to join the front of the line outside. We really didn't know we had done anything wrong.
Long story short, if it is the first meet and greet of the day, you should wait outside the tent, even if it is open and there is no one else around.