Square SMS marketing

I need somone who is knowledgeable with square sms marketing. I dont want to have to send customers request to accept to join. I just want to be able to send the messages to the clients in my contacts already. If you know how to do this without sending the text for them to join the text list, this would be super aweome. If you have any cheaper alternatives were open to that as well. We are a new business and on a tight budget.