I wrote the book on magic item creation (the Artificer's Handbook). Ok, it's optional rules, but it's a book, and it's been published, so take that for what it's worth (probably not much).
Anyway, that book has rules for gestalt items which are items which do more when in the company of others in the set than they do when they are alone.
I don't have my book handy, but I'd price the item using the spells message, and contingency, and maybe light just to be anal about it. In this case, the contingency would say "when I get a message, glow." The light would be the glow, and the message would be how the rings could communicate, and it would abide by the spell of the same name. You could increase the power of that spell, but that would cost more, obviously.
Given how contingency is such a high level spell, it would make for a fairly expensive item, and the fact that you'd have to make about 5 of them at a time, since they are a set.
You know, in thinking about it some more, you'd be far better off giving everyone a use-activated item that had like 50 charges of message in it. It would be far more useful, and a hell of a lot cheaper.