That is great! I am trying it out on my itouch right now. You sure you don't need a SMS plan to use it right? And that the other line receives it as your phone number?
The way it works, the app is tied to your phone number. All you need is the data plan. I know because I've used it on my iPod Touch.
When you start a group and invite people, your friends/family will receive a text message inviting them to install the GroupMe app. Now, they don’t have to install the app, they can just reply to the number that they received the text from.
However, it’s better with the app since you can see the texts in a bubble sequence timeline.
So all they have to do is install the app and register their number just like the inviter did. After that, they will see the group you created and any messages that have been sent within that group.
It’s a very simple and useful app. Just so you’d know, GroupMe was purchased by Microsoft, so you know they have the support.