BuddyPress.org

A long while ago, Burt Adsit wrote https://wordpress.org/extend/plugins/bp-community-blogs/ . I still use this plugin although I couldn’t tell you the last time Burt updated it. he has been awol lately. It still works as it doesn’t appear to be too dependent on the BP framework. For me, I have a blog I use as an “announcements” tool i.e. all registered users can write announcements which get displayed on a widget. So the plugin auto-registers each user to that blog as an “author”. Sounds like this is what you are looking for. Hope this helps.