Hi Frank,
One way you can do this is to add an Expression column to your GridGroupingControl where the expression accepts a value and then returns a groupID value that identifies values between 80 to 90 with the same ID, values 90-100 with another ID, and so on. Here is a KnowledgeBase article that shows you "How to create custom group in a GridGroupingControl?".
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=147Best regards,
Haneef