You can send a text message to practically an unlimited set of people. In order to do so, you have to set the Max Recipients in the settings to 10, since Windows Phone can only send a text to 10 people at a time. With the setting in the app configured, the application will send the text to the first 10 people in your list, then you can click the back button, and it will automatically select the next 10 contacts, etc.
From the application, you can set up your Google account so that your contact groups are synced into gMsg. Then, you can send a mass text message to whatever group of your contacts you choose.
The app solely uses your Google contacts, which can be a positive for those who already have a wealth of their contacts syncronized with Google. However, for those who don’t use Google, you can’t select contacts from your phone (such as Windows Live or Outlook contacts). Also, creating contact groups must be done from Gmail and cannot be created through the application. (Update) You can now create/delete groups from within the application, and add/delete individual people to/from those groups thanks to an update the developer released. And you can also call an individual person from a group instead of texting him.
If you already use Google or are willing to export your contacts into Google, gMsg is perfect for sending group text messages, and can be found in the Marketplace for $1.99.