From the main menu (you may have to to click on the Operata icon to expand the menu) select Settings and then Teammates.

The Teammates option is where you create new Teammates within the Group you are logged into.

Select +TEAMMATE to add someone.

Fill in the details, selecting the Role 'Group Admin' (can create other Teammates and has full access) or the Role 'Group User' (can view but not create anything)
Then select SAVE to create the Teammate.

More details on what the Group Admin and Group User roles can be found here..