[DefaultValue(true)]
public bool Expanded { get; set; }

<DefaultValue(True)>
Public Property Expanded As Boolean

public partial class ASPxperience_LoadOnCallback : System.Web.UI.Page {
     protected void LoadGroupContent(NavBarGroup group) {
         Control control = null;
         if(group.Expanded && group.ContentTemplate == null) {
             switch(group.Name) {
                 case "Group1":
                 case "Group2":
                     control = LoadControl("UserControl.ascx");
                     break;
             }
             group.ContentTemplate = control as ITemplate;
         }
     }
     protected void LoadExpandedGroups() {
         for(int i = 0; i < ASPxNavBar1.Groups.Count; i++)
             LoadGroupContent(ASPxNavBar1.Groups[i]);
     }

     protected void Page_Load(object sender, EventArgs e) {
         LoadExpandedGroups();
     }
     protected void ASPxNavBar1_ExpandedChanged(object source, 
     DevExpress.Web.NavBarGroupEventArgs e) {
         LoadGroupContent(e.Group);
     }
}

Partial Public Class ASPxperience_LoadOnCallback
    Inherits System.Web.UI.Page
     Protected Sub LoadGroupContent(ByVal group As NavBarGroup)
         Dim control As Control = Nothing
         If group.Expanded AndAlso group.ContentTemplate Is Nothing Then
             Select Case group.Name
                 Case "Group1", "Group2"
                     control = LoadControl("UserControl.ascx")
             End Select
             group.ContentTemplate = TryCast(control, ITemplate)
         End If
     End Sub
     Protected Sub LoadExpandedGroups()
         For i As Integer = 0 To ASPxNavBar1.Groups.Count - 1
             LoadGroupContent(ASPxNavBar1.Groups(i))
         Next i
     End Sub

     Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
         LoadExpandedGroups()
     End Sub
     Protected Sub ASPxNavBar1_ExpandedChanged(ByVal source As Object, ByVal e As DevExpress.Web.NavBarGroupEventArgs)
         LoadGroupContent(e.Group)
     End Sub
End Class