Instead of specifying each number one by one, is there anyway I can specify a group when sending sms?
For your info, I am using the contacts synced from Gmail, not the iPhone's built-in address book.
There's an app called Smart Group that might accomplish what you want to do.
When you create an SMS, you can specify the individuals to send to. Once sent you can see the group in your list of messages and send another message to that group. Unfortunately this is as elegant as the bundled functionality for grouping contacts gets without installing 3rd party apps.