The count of group members could be included on Group pages where appropriate (and if not too expensive).

Places to consider:

• Top of the "Manage Group Memberships" page OR globally near or within the title for a group above all tabs (the latter requires providing this information across multiple controllers)
• Included in the Nested Group drop-down select box in "Add a nested group"
• Next to each source and target group on the Nested tab
• As a column in the Group index view (if not too expensive).