Hello dhanesh.sakhi,
Thanks for participating in the Apple Support Communities.
If you're not able to send group SMS messages from your iPhone 6, but you can send them to individuals, try these troubleshooting steps:
- Restart your device.
- Check for a carrier settings update.
- Check for an iOS software update.
Go to Settings > Messages and turn iMessage off, then back on.
If the issue continues, you can find some additional troubleshooting steps (as well as the steps above) at this link:
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
All the best,
Jeremy