So I looked at another site and a lot of people said they had just signed up for group me....which I just did the other day. So I guess mystery solved if you just downloaded it!
Oh, I had the same thing, although I didn't know what was happening until I just texted ok to the number and I believe a bot responds with instructions. It is supposed to be GroupMe app confirmation code I think. To verify your phone number on GroupMe so perhaps somebody used your phone number as their text number or they mis-typed their own number and it happened to be yours!