Yes. When you open a Deal, you will find a section called Contact Roles that is specifically designed to let you associate more than one Contact with it, each with their own role in the deal.
Click Add Contact Roles within the Contact Roles section to bring in individuals who already belong to the Deal's Account, and to specify what role each of them plays in the deal, such as decision maker or influencer.
If the person you need to add is not already linked to the Account, click More Contacts instead. This lets you add an existing Contact from elsewhere in your CRM, or create a brand new Contact record on the spot. This is useful for people who influence the deal without being employed by the Account itself, such as an external consultant.