Re: One Blog + Community
Are you trying to keep Group A from reading the Posts from Group B? If not, what about just using the P2 theme http://p2theme.wordpress.com/ on a Group Blog using this plugin https://wordpress.org/extend/plugins/bp-groupblog/ and then instruct users to categorize or tag appropriately. P2 makes it easy for the community to post without having to go into the dashboard.
Sorry if this isn’t what you are thinking of.