Group SMS is a very much useful service. We can have groups of colleagues and classmates and then send them SMS alerts very easily. There are many online services which provide bulk SMS. But everyone of them charges per SMS or attaches advertisements with the message. But there are many service providers which provides free sms for its SIM based tariff plans.
Using that sim and GSM modem, its really easy to setup an SMS server which acts as the group SMS server. The program that runs on the server will be configured to accept messages from certain numbers. Every message from an approved number will be send to all mobile numbers in the group. Since the SMS is free, there is no cost involved .