The functionality outlined here:
https://docs.opsmanager.mongodb.com/current/tutorial/manage-users/#add-users

Does not exist in Ops Manager 2.0.4/2.0.6 (through this series of steps)

Users can only be added via a Group per Cailin's response here:
HELP-2607

Please update the documentation to indicate that users must first be added to the Group then they can be set as admins for Ops Manager.