HI There -
You have two choices. You can keep the user list on your end, and fire one transaction.send API call for each number, or you can build the user list using our list management APIs, and then do one single send to the list with the list.send API call.
I'd recommend starting with the single message, the transaction.send, because it's so easy.
You can try it out in the API explorer GUI, which is in your developer Dashboard. Just choose the option to send a message, and fill in the destination phone number. The API explorer will show you the API call, and you can make the API call there and see the response.
Regards -
Anthony
Anthony Rossano, Technologist