Create a contact card
You can create a contact card for a friend right from the Messages app. After you create the contact card, you can easily send that person a message by typing their name in the To field of the Messages window, or clicking the Add button 
next to the To field and choosing them from the list.
For more information about contacts, see Add contacts.
Create a contact card for one person
Select a conversation or someone in a buddy list, then choose Buddies > Create New Contact.
If you see Show Contact Card instead of Create New Contact, you already have a card for that person.
Fill in the fields in the contact card. Be sure to enter any instant message accounts you want to be able to use to send the person messages.
Create one contact card for someone with multiple buddy names
If you’re showing buddies for all your active accounts in one list, and the list contains multiple entries for a single person, you can quickly add all that person’s accounts to a single contact card.
Command-click all the entries for the person in a buddy list, then choose Buddies > Assign Buddies to One Address Card.
In Contacts, edit the new card to specify a first and last name.