When a person goes to register but doesn't complete the process, they are added to the failed registration group. Why people start the registration process and don't complete it can be varied (door bell goes, kids screaming or, if you've set things up so people have to pay at the point of registration, they realise they've got to part with some money!) but this group allows you to follow-up on those that have at least started the process.
Once someone is in the Failed Registration group you can view their information (to make sure it was a genuine attempted at registration) by clicking on their name and reach out to them to see they still want to register. If they do, you can approve them from the group.
For full information on Failed Registrations, click here.