A message to all members of a group can only be sent by admins, believe me you want to receive this, for example the Shoe Lovers group might soon receive a special discount code for shoes, these types of special deals are things that our team is working on setting up with Site Sponsors. You want to receive these msgs! The same goes for a message to the entire network, only admins can send these and thats how we get awesome updates coming your way!
The most common question is "How can I stop email notifications of everyone joining groups I belong to?"
The answer is to goto the group, and in the upper right hand corner click "Stop Following"
Every single email you receive from PL also contains a link at the bottom right after: "To stop following this discussion, go to:" which you can click to manage these settings.