All users will receive a Site Activity email for each site they are a member of.  However, they will only see activity that their account can see on the site (to prevent information being compromised). 

Example One

You are the site owner of your Guild Launch site.  Because you are automatically a forum moderator for all forums and are a site admin, you will see all relevant activity on your site.  Nothing will hide from you.

Example Two: Regular Member

You are a regular member of your site.  You do not have permissions to view the Officer forum or the Super Secret forum on the site.  When you receive your Site Activity email, it will not include anything from those forums, because you cannot see it on the site.

Example Three: Officer

You're an officer of your site.  You have permissions to see the Officer forum, but not the Super Secret forum on the site.  When you receive your Site Activity email, it will include content from the Officer forum, but not the Super Secret forum because you don't have permissions to see it.

Users can opt out of this email on the Communications tab of their User profile, under Email Notifications.