No User Groups show up when trying to assign a user group to a user in Control Panel > Users and Organizations. However, I am able to assign a user to a User Group if I go to Control Panel > User Groups and click on Assign Members.

1. Create a test User: usera
2. Create a test User Group: testusergroup
3. Navigate to Control Panel > Users and Organization
4. For usera, click on Actions > Edit
5. Under User information, click on User Groups
6. Click on Select, note testusergroup does not show up