Triggering email alerts based on column values


I have created an online reservation form. I now need external users to be able to check-in. My current idea is to have the internal staff trigger the update request. My issue is how to get that update request to the external user.

The visitor email field is a contact list. However, as these are external requests I cannot add persons to the contact list. The value of this field is expected to change by row. Any suggestions on how I can accomplish this or something similiar would be greatly appreciated.