"Sort" isn't really the term: it's commonly called the dedupe key or merge key.
If you want to set a different dedupe key that includes the email address + another field, you need to contact Support.
Otherwise the expected behavior is that the lead filling out a form is identified by their email address.
Why does a registrant "need" to use their email address for the additional registrants? What you're asking is actually a much larger question, not around sorting, but around the intentional creation of what Marketo would consider duplicates.
In Marketo, email address is treated as a unique identifier, and is used to map data back to the lead. If you encourage leads to fill out the same form multiple times with different person information under the same email address, they will simply be overwriting the person info attached to the record with that email address over and over again.
And, if you did deliberately create duplicates, there could be consequences down the line depending on your region's opt-in compliance regulations. It may also impact your deliverability rates, depending on how well structured your system is and how much emailing you do - note that Marketo's unsubscribe functionality (Durable Unsubscribe - Marketo Docs - Product Docs) means that if one lead with this email address was unsubscribed, all would be. All in all, multiple different people with the same email address is something to avoid.
There are other ways to enable your contacts to register multiple people for an event which, while a bit more advanced to set up, would work better. This post is one example: The specified item was not found.. There are other ways as well, including the use of additional form sections to note the additional attendee details in custom fields on the contact record of the primary registrant (this does not support the creation of additional leads, so will not show additional attendees in your reporting, but is a mechanism for capturing their data).
Hope that helps.
In Marketo, email address is treated as a unique identifier
Sorry to nitpick, but Email Address is decidedly not a unique identifier.
It's used as a dedupe key, but that only works when you're passing through a layer of application code (such as the form processing layer) before you hit the database. In such cases, there's actually a select-and-upsert process happening, where the best-fit lead is located (using several criteria) and then is updated by its Lead ID.
Once you have a Lead ID targeted, there is no restriction on the email value across the column, so in database terms it's neither unique nor primary.