I'd like to set up newsletters for three of my WPMU sites. I'd like to allow subscribers to subscribe from any site, to any site. I'm thinking a good way to do this is member groups. If I enable e-Newsletters on just one site, I'm guessing the subscriber database exists on just that one site. But if I enable e-Newsletters network-wide, is that subscriber database shared across all sites?
What's the best way to have one subscriber list across multiple sites (without using BuddyPress), but allow subscribers to subscribe to different groups?