To make someone else a manager you need to be a manager of the workspace yourself.
To promote an existing member to a manager:
- Open the Dashboard for the desired Workspace Home.
- Click the Manage Workspace cogwheel icon, and choose Members.
- Find the member you want to make a manager either by searching or paging through the list.
- Next to their name of the selected member will be an Edit Icon, click on it.
- Tick This User is a Manager, found under User Permissions.
- Click Save.
To invite a new person and make them a manager look at Sending out invitations.
Remember: making someone a manager means that they have complete control of the workspace.