Can a recipient be a member of multiple groups, for example, if I happen to be among coworkers and family (or whatever)? If so, hopefully I would get one newsletter containing all the posts tagged as any of my groups. But this means the potential number of different newsletters grows exponentially relative to the number of groups (although in practice, it would be extremely unlikely that a huge number of combinations actually exist) which might or might not matter, depending on whether each recipient already gets a bespoke copy (greeting them by name, click tracking, etc.) or if you're relying on listserv style distribution.