From the User Maintenance Manager change the User record:
From the Ribbon, click Options and then click Manager.
Click User Maintenance and select the row with the User that needs to be added to a group.
Click the lower Group tag
and then click ✚ Add.
Select the Group and ten click OK.
Click Save and Close.
The user will need to sign out of, and back into Access RDB for the changes to take effect.