How do you remove yourself from an iMessage group

My daughter has been linked in several groups for iMessaging and does not want to be a part of them anymore as they can send more than 500 messages a day which can be annoying. You cant block a group and she doesnt want to block the individuals as she still wants to message them from time to time but not be part of such a big group that has people in there that she doesnt know. Is there no way to reject yourself from a group?