Settings > Email Options
- Add 'Subject Prefix'
Permissions > Basic Permissions
- Change 'Post' permissions to 'Anyone on the web'
- Change 'Join this group' permissions to 'only invited users'
Permissions > Moderation Permissions
- Change all to 'Owners of the group'
Permissions > Access Permissions
- Change 'Contact the owners and the managers of this Group' to 'Anyone on the web'