Are there currently any bluegrass bands using this system? I am curious how many units I would need for a four +/-1 piece bluegrass/old-timey band and how many subs.

OK. Maybe I was not descriptive enough. Our group has acoustic guit.(mic'd)/vox, fiddle(mic'd), mandolin(1/4)/vox, sometimes an extra guitar(1/4)/vox and keyboard. What would be the most cost effective, but still great sounding, system for our group.

Thanks

Two systems would be a perfect start for your group. You would have four mic inputs and four instrument inputs with the two systems. You could get by with one system if you used a small mixer, but you would lose the great localization effect you achieve with two. The system will amplify your bluegrass group perfectly, and with all the new gigs you'll be getting, you can buy another two systems. Four systems will give each player their own, which is the best way to go. You will experience the utmost in localization, the same effect you get playing without amplification, and the sound is like Aural Panavision.

As far as I know, we haven't worked with a dedicated bluegrass band yet, but certainly with bands that have similar styles and/or bluegrass in their repertoire. From what I've heard there, our system that should be an excellent fit for this application.
The recommended number of system is trade off in your case. You will get the best results if every player uses one system. There are many advantages to having "your own" system that you take wherever you go. You play through it on stage, during rehearsal and event at home. It's the system you craft your sounds with. After a while the system becomes an extension or even an integral part of your instruments. That's why we call it Personalized Amplification System™ family of products.
Another advantage is that setup and tear down becomes very simple. Everybody just shows up with their own system, sets it up and is ready to play. There are no interconnection between player systems and no one needs to know any technical details about any other players setup.
The obvious advantage is that it requires a sizeable investment and commitment from all players in the band. Also if you are playing mostly on relatively small stages where floor space is at a premium, you may be better off with fewer systems. In this case you wouldn't get much spatial separation between the individual systems anyhow.
In your case 3 systems might be a good compromise. From how you described your band I guess that sharing should be very simple and should be chosen mainly by musical criteria. If you have any "lead" players, i.e. lead vocals and most solos, these should not share system but I would pair a "lead" player with a "support" player. (I use these terms strictly in terms of musical roles and certainly not as judgment about players). If you use lots of harmony vocals, these should also go into separate systems.
Hearing a 3 or 4 part harmony over a distributed stage is truly a great sounding experience for everyone.