I think you may want to consider a service such as Texthub or even consider using Twitter. Whatever route you choose, you're better off asking people to opt-in to receiving text messages rather than trying to pull this information and just send a text message to people. Remember that most phone companies in the US charge people to receive text messages if they don't have a text plan and that wouldn't be too well received.
There's a new website on using Twitter for churches (appropriately - www.twitterforchurches.com) and a lot of pretty good information out there in general. Create an account for your purpose, ask people to send a text message of "Follow xyz" to 40404 where xyz is your account name and they'll get any messages you post to that Twitter account on their phone. Of course, you'll want to double-check these instructions prior to posting them somewhere, but that sums it up pretty well.
Texthub offers some plans for non-profit organizations and is another way of using text messages to send to a group of people who opt-in. I think you'll be better off with Twitter in most cases, but TextHub may offer better support for some purposes than Twitter.
-Peter