There is no way to group them in the My Photo Stream section because this is mainly used for transferring pictures from one device to another. The photos will drop off of My Photo Stream once there is more than 1000 photos or it has been longer than 30 days. Take a look at this article: http://support.apple.com/kb/HT4486
Sometimes when I need group pictures in Photo Stream, I use a Shared Photo Stream. I just share it with my own Apple ID. Take a look at this article for steps to make a Shared Photo Stream: http://support.apple.com/kb/TS4379
I hope this helps!