Unable to invite collaborator via email

I just established my repository on GitLab, having migrated over from GitHub. I want to invite my teenage friend who does not yet have a GitLab account. So I click “Invite members” and it says I can enter a username or email address, so I enter his email address. Everything looks right… But the “Invite” button refuses to become available!

What could be wrong here? I am the owner of the repository, and properly logged in, no question about that. Do I need to separately ask him to create a GitLab account, before I can invite him? (That would seem bad for business!)

Here’s how it looks: