As I understand, you want to have aggregate functions for the groups, but not to show them in the Group Header. In this case you could create your own custom GroupRowStyle, delete its declaration of the field for aggregate functions and use it as a Resource for the grid
In that way your own functions will appear only in the GroupFooter (the property of the grid "ShowGroupFooters" has to be set to "true"). Here, the function I use is "total". Furthermore, in case you want to show the function for the whole grid, not only for the group, you should set the property of the grid "ShowColumnFooters" to "true".
I hope this will help.
the Telerik team
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