Group SMS - HELP! Thank you.

[Capitan Totti]

I have an iPhone 3G on iOS 3.1.3, T-Mobile.
I would like to enable Group SMS.
Basically, when my friend send a Group SMS to me and other friends, the ones who have iPhones get the message, and once they reply, all of the participant get that message. If I reply, I only reply to the main sender.

Is this an option available only on 3GS and up? Or maybe AT&T?

Thank you.

[Capitan Totti]

Anyone?