In that case, you will need add the field using a template as demonstrated in the provided link in my previous post, or preserve the corresponding value on a ViewState or Session variable and use it during the ItemDataBound event handling for the GridGroupHeaderItem.
Hope this helps. If you need further assistance, please elaborate on how you are getting the condition value or send us a very basic runnable sample demonstrating your specific scenario. Thus, we will be able to further analyze the requirement and suggest a more-to-point approach.
the Telerik team
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