I don't think you have a lot of options here.
If you want the guest validated before getting 'full guest access' you're going to need to have someone validate their account.
This can be done either ahead of the visit by having a guest-provisioning operator create the account (often the receptionist) or during the visit with something like sponsor verification.
Or you give everybody in the company guest provisioning rights and an instruction booklet detailing on how to create guest accounts themselves :P