Salesforce allows you to associate contacts to opportunities via the contact role section.
To associate a contact with an opportunity, follow the directions below.
1. Click into the opportunity you wish to link the contact to.
2. Scroll down to the Contact Roles section.
3. Click "New" to edit or create new Contact Roles.
4. If the contact is already listed, designate the role that the contact plays in the opportunity.
5. If the contact is not listed, click on the search icon and select a contact. After selecting the contact, designate the role that the contact plays in the opportunity.
6. If the contact that you are associating with the opportunity does not exist in salesforce.com, click on the search icon to the right of the contact field and click the create a new contact link.
7. Click "Save" to complete the association.
Note: A contact is automatically associated to an opportunity as a contact role if they are created at the same time due to lead conversion