Steps to reproduce
==============

1. Create a new course.
2. Enrol some users.
3. Create some groups and assign the users.
4. Make a backup. Keep 'Include enrolled users' checked.

Expected result
===========

Even if there is no group related activity yet the groups memberships of the enrolled users should be backed up.

Actual result
=========

No groups in the groups.xml file.

Proposed fix
=========

Explicitely check for 'annotated' users in class backup_groups_structure_step in backup/moodle2/backup_stepslib.php. See attached patch file.