I don't have a problem with it, as long as the group that joins gets 1 or 2 pictures. What I have a problem with is when Dad waits in line and then Mom and 6 kids join at the last minute, and then each kid needs their own autograph and picture, then just a picture of the girls together, then the picture of the boys together, then just mom and Susie, etc.
However, my husband and I have already agreed that if our DD wants to do something, she needs to wait in line for it. Waiting is part of life and she needs to learn. She also needs to learn about making choices and consequences, like if I choose to wait for A I might not get to do B. Besides, why should she get to have fun with one parent while the other is stuck standing in a hot crowded line?