An admin can’t be added to a Workspace directly. Instead, they must be invited to the Workspace as a normal member, then changed to an Admin.
To change a member to a Workspace admin:
Click Members in the sidebar to open the Workspace members page
Click “Normal” next to their name, then select “Admin” from the dropdown.
All members of a Free Workspace are admins of the Workspace. However, Free Workspace admins are not given access to all boards in the Workspace. Individual boards must be managed by the board admins.
On Standard and Premium Workspaces, Workspace admins are also board admins on all boards in the Workspace, even private Workspace boards.