You cannot send an SMS to more than one number in one request. If you are using the HTTP API you will need to loop through your numbers on your end and make a request for each number if you want to send to a group.
While you cannot send to a Group you have created in your account, you can use our REST API to send the same message to multiple phone numbers passed to use as an array. See The REST API documentation here.
For complete API info, click here.