GroupMe on Windows Phone - how to integrate to built in messenging app?

Fun fact: groupme is owned by Microsoft. One problem though is that on my android device and also on my old WP 7.5 device I could get groupMe messages via text message, through the built in messaging app. For some reason, now it only goes through in the actual GroupMe app - while not bad - is slower and less enjoyable than simply messaging groups with the built in messenger. How do I get it to send to my messages? I don't want to use the app. I had it before, and I think it was on default, but not now....