For rides like that I think they need to implement a rule stating if you don't enter the line with your full party and/or don't keep your party intact while in line, you don't get to re-enter the line or have people meet up with you. It's just plain rude, imo. Especially with large groups.
For meet and greets, especially the Frozen one, I suppose it does make sense as long as the full party enters the line and then splits. That way people aren't surprised when more people enter your group near the end of the line.