By default, RadGridView's group headers will go out of view once you scroll past them as illustrated in Figure 1.
Figure 1: RadGridView with Disabled Sticky Group Headers
You can stick the headers of the group to the top until its items are scrolled out of view by setting the EnableStickyGroupHeaders property to true.
Example 1: Enable Sticky Group Headers
<telerik:RadGridView EnableStickyGroupHeaders="True" />
Figure 2: RadGridView with Enabled Sticky Group Headers