This feature is not available upfront and one need to activate the same by going to “Labs” section in Account settings. You will have to enable SMS service which will help you send sms to any person who might while chatting may get out of the internet connectivity.
AS Per Google explanation regarding the service it reads as following:
SMS credit is an allocation of SMS (text messages) that you're able to send to a mobile phone:
"Buying" additional messagesKeep in mind that if you'd like a higher message credit, you can always send an SMS to your own phone, and then reply to that message multiple times. Every time you send a reply message, your SMS credit is increased by five. Effectively, you're buying more messages by paying your phone company for these outgoing messages.
Hope now you will not be out of communication even if the other party has gone off the internet connection and this service of a great use during chat when the other party gets out of the internet connectivity. You can send sms to a group by using the Google Free SMS channel and be in communication with your group free of cost.