You can do something like this with FriendFeed. We use it for a programming user group. The feature is called 'FriendFeed Rooms' (http://friendfeed.com/rooms/overview). You can create a room for each group (public or private). Use the bookmarklet (http://friendfeed.com/share/bookmarklet) to post links to any room, as well as add your own comments to the link. Users can get their FriendFeed mailed to them, if they wish. Or, you can make the room public, and they can visit it at their leisure, without signing up.

I recommend giving FriendFeed a second chance as an app, in any case. Every once in a while I come across another feature that I wish FriendFeed did that it does, indeed, already do. It does a ton, and it does it well.

EDIT: I missed one of your needs: Yes, one can subscribe to a FriendFeed Room's feed in a feed reader (that reads Atom-formatted feeds). It can also be read via Facebook.