...
Click on "Users" at the top menu to go proceed to the user account management section.
...
In order to be able to delete a user group, it must not contain user accounts. Move all users of the deleted group need to be removed to another group, then select the deleted group and click the "Delete" button.
...
Select the account to which the device groups should be assigned. A block with information about the selected group and management options will appear on the right. Go to the "Devices" section where all the assigned device groups are displayed. To assign a new group, click on the "Add groups" button and in the new menuwindow, check all the groups that should be assigned to the selected user account.
To deselect, click on the corresponding device group again. To apply the changes, click "OK" and then "Save".