Cannot add an eligible user as team member

[jakepeyser]

I am trying to add user @brdwrd as a team member in the developer portal, but I receive a “Invitation can’t be sent” error telling me that they are not eligible for a team invitation.

I have read through the linked documentation and confirmed with the user that none of the criteria that prevents them from being added applies to their account. Is there some other reason why they cannot be added?