In Moodle 1.8 it appears that the view by 'separate groups' feature has been inexplicably dropped, or there is a bug.

In 1.6, assuming the teacher had divided enrolled students into groups, it was possible to select 'Separate groups' in: the Gradebook, Quiz results, Workshop, and Assignment to view the results or activity of students according to the groups to which they belonged. However, this does not appear to be possible in 1.8. Although, it is still possible with the Forums, Participants (the link in the People block) and Reports (the link in the Admin block). The inability to do this is odd since the set up pages for these activities has the 'Group Mode' choice of 'No groups, Separate groups, Visible groups', yet it is not in fact possible to view the activity of students based on the groups to which they have been assigned.

I realize that the concept of Groups has changed a little in 1.8 (http://docs.moodle.org/en/Groups#Groups_in_Moodle_1.8_onwards), but it seems to me that the inability to view separate groups in various activities and the gradebook is a big problem for people with large numbers of students in the same course.

This report relates other bug reports:
MDL 8564
MDL-8735
MDL-8949
MDL-8952
MDL-9081

This bug / issue was observed in Moodle 1.8 downloaded on the 2nd April, 2007.