Maybe the best way to achieve this is for all Employee and Alumni ranks to set them up that they revoke the "Registered" role (which you hopefully have set up on your community for all accounts that finished their initial registration). Then you could simply use this registered role in this query. But there's a bit of a long tail of possible side-effects when going for this approach as some components and access permissions might be set up on your community to check against Registered role.
Can't think if a better way right now sinceyou still cannot query for "does not have role" in LiQL... 😑
