I do this all the time, if I'm understanding you correctly.
I create the SMS, and then add recipients. I don't know that I've ever tried to add 50, so I'm not sure if there's a limit. But, I've sent 20+, and had no trouble.
I don't know how to create a group, if there is a way. I have to do manual addys every time. But, it IS possible to send the same text to multiple people. I couldn't do that on my 7290, so I think it's new to the 8700.