Pop up Panel, Postback problem

I have a panel as a pop up

 <asp:Panel ID="CreateGroupPanel" runat="server" BackImageUrl="~/Light-Gray-Suede1.jpg" 
        CssClass="style10" Visible="True" 

        style="position: absolute; left: 214px; top: 1262px; width: 588px; height: 467px">
        <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
         </asp:ToolkitScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">

         <ContentTemplate>
        <asp:Label ID="Label3" runat="server" CssClass="style11" Text="Group Name*" 

            style="position: absolute; left: 63px; top: 92px; width: 88px; height: 19px; background-image: url('Light-Gray-Suede1.jpg')"></asp:Label>
        <asp:Label ID="Label4" runat="server" CssClass="style12" Text="Members*" 

            style="position: absolute; left: 67px; top: 278px; width: 66px; height: 19px; background-image: url('Light-Gr开发者_运维百科ay-Suede1.jpg')"></asp:Label>
        <asp:Label ID="Label5" runat="server" CssClass="style9" 
            Text="Groups | + Create group" 
            style="background-image: url('Light-Gray-Suede1.jpg')"></asp:Label>
        <asp:Label ID="Label6" runat="server" CssClass="style13" 
            style="background-image: url('Light-Gray-Suede1.jpg'); position: absolute; left: 63px; top: 130px; width: 80px; height: 19px;" 
            Text="Add user(s)*"></asp:Label>
        <asp:TextBox ID="GroupName" runat="server" CssClass="style109" 


            style="position: absolute; left: 170px; top: 91px; width: 135px; height: 22px"></asp:TextBox>
        <asp:Button ID="Create" runat="server" CssClass="style106" 
            onclick="Create_Click" 
            style="background-image: url('Light-Gray-Suede1.jpg')" 
            Text="Create Group" />
        <asp:Button ID="Cancel" runat="server" CssClass="style107" Text="Cancel" 
            onclick="Cancel_Click" 
            style="background-image: url('Light-Gray-Suede1.jpg')" />
        <asp:Button ID="Add" runat="server" CssClass="style108" OnClientClick="Add_Click" 
            style="background-image: url('Light-Gray-Suede1.jpg')" 
            Text="Add" />

        <asp:ListBox ID="ListBoxMembers" runat="server" CssClass="style105"></asp:ListBox>
        <asp:ListBox ID="ListBoxREmoved" runat="server" CssClass="style104"></asp:ListBox>
        <asp:Button ID="Button1" runat="server" CssClass="style103" 
            onclick="Button1_Click" Text="Remove&gt;&gt;" />

        <asp:Listbox ID="ListBoxAddMembers" runat="server" CssClass="style102" 
            DataSourceID="SurelyKnown" DataTextField="FirstName" DataValueField="FirstName"></asp:ListBox>

    <asp:SqlDataSource ID="SurelyKnown" runat="server" 
            ConnectionString="<%$ ConnectionStrings:surelyknownConnectionString %>" 
            ProviderName="<%$ ConnectionStrings:surelyknownConnectionString.ProviderName %>" 
            SelectCommand="SELECT FirstName FROM tbl_user"></asp:SqlDataSource>

    </ContentTemplate>
     </asp:UpdatePanel>


</asp:Panel>

When i click add button inside the panel to add values from list box to the text box, the panel gets post back because of the add button click and the pop up is closed because of that.(but when I again click the button to open popup the Add method is performed and the value is added) how to add values to text box from the list box without the pop up being closed

Try setting ChildrenAsTrigger="false" for the update panel and specifying explicit triggers.

If you haven't already, I would put the Panel and the ModalPopupExtender (assuming that you're using AJAX Toolkit) in an UpdatePanel, and as Bala R suggested try setting ChildrenAsTrigger to false.

Using an update panel, even if the button click event still causes the popup to close, you can reopen it in the button click event, and it will appear seamless to the user since it's all being done via Ajax.