To make a workspace's member a Subject's officer, go to the subject page and click on 'Manage members' on the top right of the screen. 

This will display a window listing all subject members, where you can change your team members' role from member to officer by clicking on the gear icon and selecting 'Make officer'. 

Similarly, you can turn a subject officer to a simple team member by clicking on the gear icon and selecting 'Make member'.