I found this while doing some research. It focuses on flexible grouping for reading but it does a great job simplifying the concept for any use of flexible grouping in the classroom. I think the charts are great. You might want to take a look at it.
http://www.learningpt.org/pdfs/literacy/flexibleGrouping.pdf
http://www.teacherspayteachers.com/Store/Kasha-Mastrodomenico uses flexible grouping in differentiated instruction activities, Check it out!
Written by
Kasha Mastrodomenico
http://www.socialstudiesdifferentiatedinstruction.com
kasha@socialstudiesdifferentiatedinstruction.com
http://www.teacherspayteachers.com/Store/Kasha-Mastrodomenico