Please take a look at the attached and let me know if it is possible to display a text box besides the groupby colum within RadGrid.( shown in red)
User gets to enter this value in the textbox upon which a validation needs to occur.
You should be able to add any controls in the GridGroupHeaderItem. You can do that in the ItemCreated event. Attached is a modified version of this code library where you can see how a CheckBox is added in the GridGroupHeaderItem.