In the group context, I think this makes a lot of sense. It is similar to Path (which is basically a "group" limited to my closest friend). For every post I make, it shows me which of my friends saw it, and that is convenient and cool.

I think this could work well for small groups of trusted friends (or shared interest groups). For large scale groups of strangers this could be creepy though. I definitely would hate to see it abused by larger brands, etc (or for this to be integrated into the main news feed for that matter)... which unfortunately at this point I can't say I trust Facebook enough to believe they wouldn't try to push it past that limit