I have observed that there are two unique identifiers for Contacts: email and Record ID. My inquiry pertains to how the email address can be added to Contacts in cases where multiple clients share the same email address.
You’re correct, an email address can only be held by one contact.
The same email address cannot be used across multiple contacts. The email address is the unique identifier in HubSpot. Whenever a form is submitted or data imported, for example, HubSpot deduplicates by email address.
One option would also be adding these contacts without email address and pinning a note to the contact record. This note could include the email address or simply state that all contact from the company are sharing the same email address.