If you wish to add an existing contact into more contact lists go to Manage Contacts in Contacts and find your contact. Mark it and click on Edit lists to the right to select what lists to add the contact to. You may also remove it from contact lists or create new lists with that specific contact in it.

If you mark more than one contact select the Edit lists button above the list to edit them all at once.

Please also see:
How do I add contacts?
How do I download my contact lists?

Or learn more about Contacts in our Guides