First, I do not believe that the term crowdsourcing means what you think it does. What you are describing is more just group texting and group alerts. To do this, you could either subscribe to a service to do it for you (easy, efficient, and guaranteed) or you could build something yourself.
If you really want to create something for yourself, you will need to look into utilizing a service to send and receive SMS messages. One service that I am familiar with (and love) is Twilio. They offer these types of services on a global scale. They also provide sample code to connect to their service from code. Here is a link with demonstration code and other info:
You are going to need to lay out what you want to do, break it down into small parts, and start building your app from there. Then you will either get your complete application or run into specific problems that we can more easily help you with.