If your sending via SMS/mms you can just use email and BBC each number , or alternatively you can toggle send as group message in your messaging app and when you send a group message it will send out one to each recipient
I feel email is the way to go since if they text you back you can set up a rule to email all replies to you phone via sms. Its a great way to keep replies under one chat.
Note this method is not secure as not all providers encrypt these transactions, but if your using SMS anyway and what your send is not sensitive you might as well