How to display all employees under each manager

I have a 2 tables:
Mrg: 
UserID || UserID_Mrg || Status
and 
User:
UserID || Name 
I want to display all employees (UserID) under each Manager (UserID_Mrg)
example:
UserID_Mrg = 12
So display all UserID with UserID_Mrg = 12
so far I could just display the users Information an their Manager:

<pre lang="c#">private void BindGridView()
    {
        using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conn"].ConnectionString))
        using (SqlDataAdapter adapter = new SqlDataAdapter(@"
SELECT tbl_user.Name, tbl_mrg.UserID_Mrg FROM   tbl_mrg 
INNER JOIN tbl_user ON tbl_mrg.UserID = tbl_user.UserID 
WHERE  ((tbl_mag.user_ID = @UserID))
ORDER BY     tbl_user.Name", con))
        {
            adapter.SelectCommand.Parameters.Add(new SqlParameter("@UserID", SessionHelper.UserID));
            adapter.Fill(Data2);
            GridView2.DataSource = Data2;
            GridView2.DataBind();
        }
    }

<asp:GridView ID="GridView2" runat="server">
                <Columns>      
<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name" />
<asp:BoundField DataField="Mrg" HeaderText="Manager" SortExpression="Manager" />

                </Columns>
            </asp:GridView>