Try a app called HandCent SMS from the Market, It has what you are looking for, Not only is it an amazing messaging app but if you want to send to a lot of people at once there is tons of features to accommodate you. With it you can create your own groups, and when you go to create a message and click on the "TO" field it gives you several options it shows the list of your contacts with boxes to select which ones you want and you can just scroll down the list selecting which ones you want or select groups that you created. It only takes a few seconds to send messages to everyone on your list or just a few. I have tried several messaging apps and this is the best one so far, It has a lot of extras to from different font packages to window styles. If you want to give it a try you won't be disappointed