User Management - Can’t change user’s usergroup

When trying to change a user’s usergroup from Users->Users->(userpage) and clicking Save User 

Sometimes the following error occurs: 

This means the user you are trying to change is already logged on, and so cannot have their usergroup changed while they’re logged in.

To change the usergroup, have the user log out. Then when the usergroup has been changed, the user can login again.