Nested GridViews - get ID from parent GridView

Hi.
Would greatly appreciate help on getting ID from parent GridView.
I have a simple db, with two tables:
Group (GroupID - primary key, NameOfGroup)
Position (PositionID - primary key, GroupID - foreign key from Group table, NameOfPosition)
I would like to be able to show in one GridView's row all positions associated with one GroupID, as shown in picture bellow (Parent GridView would have Group ID, Name of Group columns and column with Nested GridView containing Name of Position column):

Here's work in progress .aspx:
<asp:GridView ID="GridViewGroup" runat="server" Height="50px" AutoGenerateColumns="False" AllowSorting="true" DataKeyNames="GroupID" SelectMethod="GridViewGroup_GetData" ...