If you already have a Person listed in your CRM, you can link the existing Person to any Opportunity record. This prevents you from having to enter client details twice.
From the Opportunity >> Contacts section, click on the Link existing contact button.
Search for the client's name, highlight, and click Add.
You can also attach related Person records as non-Applicants. Use the drop down to select the Relationship Type.
Any Person which is not an Applicant or Guarantor will appear in the Other Contacts section.