1. Go to onMessage
2. Approval
3. Event Registration
4. Search for the Event and click Edit
5. In General Info check off the Box to "Allow Authenticated Users to Register Multiple Times".
6. Save
7. Copy the Registration Link found in General Info into a browser tab and register on behalf of those who have submitted their Event Registration through the mail. 

Once this is done these Registrants will show under the Registries Section of the Event Registration.