According to the Atlassian University course on Next-gen (Team Managed) project templates, a Project Administrator can send email invitations to unlicensed users to join projects.
Can this permission be revoked?
On our site, we have limited licences available so can not afford to have licences not managed by our Site Admins. It would be a deal-breaker for using the Next-gen projects if the Admins can not retain control over licencing.