Newly Registered Users added to group cannot be demoted or deleted

[jkmarx]

Specific code commit: fdf35ba
Version of the web browser and OS: Firefox 66.0.3
Environment where the error occurred (Vagrant VM and site conf mode or AWS instance):
https://test.stemcellcommons.org/ Also reproducible locally

Steps to reproduce

Please list all the actions and the input data used:

1. In the admin interface, add a new users and add them to the test group
2. Promote
3. Demote

Observed behavior

angular.js:12185 DELETE http://192.168.50.50:8000/api/v2/groups/6997f024-3c93-4ad3-a820-7bb3840xxxxx/members/10/?format=json 404 (NOT FOUND).

Expected behavior

• Should be able to remove/delete

Not an issue with older existing users.