Most people would not bring along a group of uninvited strangers to a dinner party or, even worse, a wedding. Society has certain expectations around attendance, guest lists, RSVPs, and the like.  And yet, in the digital realm, these social norms may not have the same effect.   What can be done about digital party crashers?  In particular, how can the owner of a social network ward off competitors who seek access to network content by riding users’ coattails?