There are various online services that allow you to send SMS/Group SMS to any mobile number across India (irrespective of service provider it may be Airtel/Idea/Vodafone/Relaince). Here Group SMS means you can send the same message to multiple persons at the same time. So to save money, save time texting and to send SMS to multiple people you can reisgter to any of the sites mentioned below, all of them provide this service for free but ypu need to register on them to use this service.
1) Indyarocks – send up to 80 chars
2) Atrochatro – send up to 140 chars
3) MyCantos – send up to 140 chars
4) 160by2 – send up to 80 chars
5) Smsze - send up to 148 chars
6) Way2SMS - send up to 140 chars