With respect to the crashes on Macs that have been around since roughly September 3rd, try installing the latest Steam beta update, in accordance with the following steps, please:
http://steamcommunity.com/sharedfiles/filedetails/?id=182912431


If the above doesn't help, send us a support ticket including the following:

 
  1. A detailed description of the problem. If message pop-ups, error codes or other notifications are displayed, copy their content or attach a picture (screenshot), please.  
  2. A System Information report generated on your system.  

  Here are the steps for you to follow in order to generate the report:
  • Open the "System Information" application
  • Press the "System Report..." key
  • Use "File -> Save..." or "⌘S"
  • Save the generated report on your drive