I like almost all these suggestions and will add them. The one II concur with Duane and J. It is a notification when sent to a specific member, or sent to a member on joining, but I think not when it is posted monthly -- then it serves as a reminder.
Posting it to the group itself aids web-reading members by exposing it not only in the group's Messages pages, but also in the compendium of topics in the members' Your Groups page.
... I feel like the archives should just contain group discussions. AmIf not wrong then at least conflicted, there is the other example of Event reminders.
Granted those may be more likely to serve as a prompt for a discussion (birthday wishes and memories as replies to a birthday announcement is common in my classmate groups), whereas in many groups a discussion of the Guidelines might be considered off topic. Still, some groups might welcome discussion in reply to a Guidelines post. For example, in my classmate groups I don't plan to prevent it -- we're pretty informal and open-topic in our discussions.