Dream Heights: How Do You Move A Friend From Facebook Into Your Own Tower?

I have read that your tower neighbors (facebook friends) can move into your tower, but everytime I go to move a friend in, it says, "You've moved in all the ones who are already playing".  I'm confused.  I invited friends from facebook, so they started playing and built their own towers. Now they are my tower neighbors but i wanted them to live in my tower so I can train them.  None of them live in my tower and It will not let me add them.  :(  Any suggestions?