Adding a user to a group is not reflected in the UI

Steps to reproduce:

1. Navigate to the groups of a user. Users -> [user] -> User Groups

2. Click the "+" button next to a group to which they are not a member.

Expected results:

• The user is added to the group, and the page now shows the groups the user is a member of.

Actual results:

• The user is added to the group, but the page does not reflect this. Refreshing the browser does not help

Note: The user /is/ part of the group, and can be seen when viewing the users in that group i.e. Groups -> [group] page. It is only the groups for that user that is not working - I suspect a cache somewhere needs updating.