How to link one contact's address to be the address of all contacts in a specific group via SQL?
This is how you set the address of all people in a certain group (group id = 13), to be the address of a specific, different contact (address id 15432)

SET @InGroup:=13;
SET @MasterAddress:=15432;

UPDATE civicrm_address SET master_id = @MasterAddress
WHERE contact_id IN
    (SELECT contact_id
    FROM civicrm_group_contact
    WHERE status = 'Added'
    AND group_id = @InGroup)