The contact will need to have logged in to the community (nation.marketo.com) at least once for their profile to be created and recognized in the system.
"In order to appear on the list of available contacts, the user must have clicked Community (while logged into Marketo) at least once."
https://docs.marketo.com/display/public/DOCS/Manage+Authorized+Contacts+in+the+Community
Once the contact has logged in to the community, the authorized support admin should be able to see the contact to be added as an authorized support contact.