Navigate to Posting Settings tab under the Post > Manage Posts top navigation menu.
In DEFAULT POST SETTINGS section check Email Footer
Once you have filled out this footer text this will be the default footer for your entire network, unless you create a custom footer at the group level, that footer will override the network footer for that specific group (see how to define custom email settings at the group level here.)
Note***: Some organizations may not want to publicize the unsubscribe url, group email address and topic link (takes users to the Regroup website). For Emergency Alert Groups it is recommended to remove the unsubscribe url, the group email address and the topic link.