One very common business case is the necessity of reassigning an existing user to a different group.
Now, the high level user roles have the possibility to lower level users to a different group.
Note the group can be changed for an existing user from the same group level.

In order to to that, first you need to click on "Staff" in the left menu:

Within the staff page, search for the user you would like to change the group and once you find it, click on it. Note: you can search either scrolling down the list or using the search functionality:

Click on the edit button in the top right of the screen:

Under group, select the new group in the dropdown and then click on the save button also on the top right:

You should be able to see the new group displayed now and the user should have the access accordingly: