Crystal Reports Group Name Fields Dynamically Change

I have created two aspx pages where first page contains a dropDownList control :
<asp:DropDownList ID="ddlGroupBy" runat="server" >
                <asp:ListItem Selected="True" Value="Department" Text="Department Wise" />
                <asp:ListItem Value="Grade" Text="Grade Wise" />
                <asp:ListItem Value="Team" Text="Team Wise" />
                <asp:ListItem Value="Category" Text="Category Wise" />
                <asp:ListItem Value="Location" Text="Location Wise" />
            </asp:DropDownList>

And a button control which redirect me to second page wehen I click it.
Second page has CrystalReportViewerControl which displays .rpt files records.
I have also taken a CrystalReport.rpt file and designed it. In this file Recoeds will be grouped by default Department Wise. But I dont know how to change the Group Name Fields (which is Department by default) if user change the dropDownControls items appropriately.