Address Book > My Contacts offers a private area to store individuals' contact information. The individuals you save in My Contacts can be linked to multiple appointments (e.g. a therapist you work with on many cases)—see Involved Persons Tab. Click the blue Add Contact button to save a new contact here. You can check a box to mark a "Favorite" and later filter your My Contacts list for "Favorites" for easy access to your most-contacted individuals. After creating a Contact, be sure to click "Save." On the My Contacts screen, check the checkbox beside an existing individual to edit or delete that entry.
While creating Involved Persons within appointments, you will also have an option to "Save this Involved Person to My Contacts" on the fly to store any contact for reuse.