I've never used BBM, but my interpretation was that it was more like an IM/text than a chatroom. If you send a person a BBM message, doesn't it go straight to their phone?No data plan I've ever heard of offers free texting. Why give it to you for free when they can charge you $.30 a pop for them?
I also don't see the point of needing a BB messenger group for MNMazda. You have a BlackBerry, which means you have a data plan...just use the chatbox maybe?