When we talked about getting Conditional Activities into 2.7, I remember explicitly having a discussion about not including group/grouping restrictions into it because of the potential for problems and confusion.
I was just playing with a Workshop in Moodle and noticed that:
a) group restrictions are in there after all,
b) despite what it says in http://docs.moodle.org/27/en/Conditional_activities_settings#Group it is not controlled by enablegroupmembers setting because mine is off and
c) it can break Workshop. Workshop is NOT aware of this restriction, and so it blithely tries to include people from all the other groups. This of course breaks the peer grading.
The solution may be a mix of CA, Group and Workshop fixes (and probably other activities as well) but we need to clear all this up.