Enabling member groups and subscribers across sites?

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?