How do I add a buddy to more than one group in OS X Messages.app? When sorting buddies for XMPP services (not iMessage) into different groups in the contacts window in Messages.app for OS X I can click and drag one buddy from one group to another. However this removes the 'buddy' or contact from the previous group. How do I add a buddy to two groups?
The Messages (Mavericks): Use buddy lists Apple support article explains how to add a contact to more than one group:


  Add a buddy to more than one group:   Press Option and drag the buddy
  to each additional group.


Hat tip to @DaveDelong.