How do I add or edit administrators?

You can edit administrator accounts at any time. Go to https://deploy.apple.com, sign in using your Apple ID and select Admins on the sidebar.
Detailed instructions can be found here. https://www.apple.com/mx/business-docs/DEP_Guide.pdf

How do I assign, unassign or disown devices?

After an order ships, you can search for the order number in the DEP portal and use it to assign devices to our Secure Device Manager. You can also unassign devices.
Or if devices have been sold, lost or stolen, you can disown them in the portal.

You can go to https://deploy.apple.com , sign in using your Apple ID, and select Device Enrolment Program in the sidebar, then select "Manage Devices".

How do I link my Apple Device Enrolment Program (DEP) device/s to Secure Device Manager?

Apple Device Enrolment Program can only be managed via our standard Secure Device Management product. If you have Secure Device Manager Basic, you will not be able to manage devices via Secure Device Manager.
The Secure Device Manager console help section contains detailed information around the Secure Device Manager link with Apple DEP.