Be sure it is an actual CM with an actual sign. Also, know that there are mornings that they do not have the sign and do not lead you to the meet and greet. We did it last Monday (7/14), a non EMH morning. They did not lead a line down to the meet and greet that morning. There was a guest down at the rope by the castle that everyone thought was a CM who was holding up an "Anna and Elsa line starts here sign", so people were lining up behind him. A real CM saw what was going on and came out and announced that there was not going to be any organized line that day, so the crowd got irritated at the guest who they thought was a CM and he put his sign away.
There does not appear to be any rhyme or reason to whether Elsa is there at rope drop (or during EMH). Some mornings she is, some mornings she comes out 30 minutes later, and some mornings she comes out an hour later. I still think it is best to make the morning run and gamble on it, and then just let people go in front of you until she shows up if it is a priority and she isn't there yet.
Making the run was easier than I expected, but just FYI we thought DL had fairly low crowds that Monday, so we might have gotten lucky and just picked the right day. We did A&E Monday morning for regular hours, thinking that if it was too packed and we didn't make it before the line ballooned up to 2 hours or more, that we would try again during early hours the next day. DH got to the park a few minutes after 7 and was the first person at gate 14DD and I arrived just after 7:30 and were only a few people behind him in line, and the people between us were nice and told us to just go up there with him. Once they opened the gates, DH booked it down to the castle with some Dads making the run (go to your left- the side closest to adventureland etc.). DD and I did a fast walk and we were not far behind him at all. They have been doing a line up behind a CM, but our morning they did not as set out above. After the announcement there was no line was made and everyone realized the guest with the sign was not a CM, we disbursed along the rope. DH, DD and I were all at the front of the rope. They drop the rope (at least our day) from the inner side which a CM told DH so we knew right where to stand. When the rope dropped everyone raced to the meet and greet. DH could have been first in line, but he decided to let a few of the running dads in front of him as he knew DD and I were not going to run. DD and I did a pretty fast walk, and even just doing that we would have been within the first 10 groups in line, not far back at all. DH was about 3rd or 4th. Unfortunately, Anna started out by herself (the previous morning, it had been Anna and Elsa together we heard from another guest who had done the run- and from yet another guest we heard that another morning Elsa had come out 30 minutes after opening). We just stood back and let people go in front of us. We hear that she usually comes out either 30 minutes or an hour after Anna if she is not there at opening. Unfortunately for us, it was an hour, but we stuck it out as DD wanted to meet both of them and we weren't doing it again. If we had not waited for Elsa, we would have been in line probably for 5 minutes. I would say that most people that were there before rope drop and even just walked to the meet and greet did not have much of a wait. It was a low crowd park day, and the line didn't really build up for about half an hour. It was an hour or even less for at least the first half hour the park was open. Not too long after that, it hit 2 hours and hovered there the rest of the day. It was a lot easier than I expected (other than having to wait for Elsa, which we knew was a possibility), so I guess we just hit it on the right day maybe. We kept an eye on the line as we wandered by the days we were there (if it had been short as a fluke, DD probably would have wanted to stop by), and it pretty consistently stayed at 2 hours most days and times. I was surprised it didn't go higher, as I have heard lots of stories of 3, 4, and 5 hours lines, although more from WDW than DL.