How Do I Invite Someone to Be a Core Team Member if They Are Not Registered with CueDB?

To add someone as a core team member, they first need to be a CueDB member. If they're not, your first step should be to invite them to a project or suggest they register as a CueDB member. As soon as you've done that, you can add them to the core team. Remember, core team members can only access the projects to which they have been invited. Only the Team Owner is granted automatic access to all projects within the team.