I did this meet and greet a few weeks ago. It is expected that one person holds the place in line for their entire group. There is a CM who asks each person who gets in line how many are in their group and they keep a tally. When that tally gets to the limit they tell the next people getting in line that they will be part of the later grouping.
We were at the 8 am rope drop and the first Rupunzel M&G was at 9:15. We got to the line at about 8:10 (had to stop to purchase autograph books) and the CM asked how many would be in our group. I told her we had 8 people and she said we were the last people for the 9:15 grouping. My wife then took our kids to ride Dumbo and Peter Pan while I held the place in line. Everyone who was in front of me did the same thing. The people who got in line behind me were told that they were in the 10:15 grouping, and the CM's did the same thing (asking how many were in each group and keeping a tally). When you get in the line they tell you what time your group would get in so there are no surprises. If 15 people join a group in front of you, they were already accounted for by the CM when you were told what time you'd get in.
When our group went in at about 9 o'clock the 10:15 group was already filled, and the next people getting in line were told they would be in the 11:15 group.