This code sample uses a default Outlook folder:

Sub Whatever()
  Dim Ns As Outlook.NameSpace

  Set Ns = Application.GetNamespace("MAPI")

'use the default folder  
Set Items = Ns.GetDefaultFolder(olFolderCalendar).Items

'do whatever

End Sub

To use a folder at the same level as the Default folders (such as Calendar, Inbox etc), use this in place of Set Items = Ns.GetDefaultFolder(olFolderCalendar).Items, where SharedCal is the folder name:

Set Items = Session.GetDefaultFolder(olFolderCalendar).Parent._
     Folders("SharedCal").Items

When the folder is a subfolder under the default Calendar folder, use this instead:

Set Items = Session.GetDefaultFolder(olFolderCalendar)._
  Folders("SharedCal").Items

To use the currently selected folder, you'll need to use:

Set Items = Application.ActiveExplorer.CurrentFolder.Items

Use a folder in another pst or mailbox

To use a specific folder in another data file in the profile, you need to use a function. Call the function in your macro in this manner, where GetFolderPath is the function name:

Set Items = GetFolderPath("New PST\Test Cal").Items

After adding the function to ThisOutlookSession:

Function GetFolderPath(ByVal FolderPath As String) As Outlook.Folder
    Dim oFolder As Outlook.Folder
    Dim FoldersArray As Variant
    Dim i As Integer
        
    On Error GoTo GetFolderPath_Error
    If Left(FolderPath, 2) = "\\" Then
        FolderPath = Right(FolderPath, Len(FolderPath) - 2)
    End If
    'Convert folderpath to array
    FoldersArray = Split(FolderPath, "\")
    Set oFolder = Application.Session.Folders.item(FoldersArray(0))
    If Not oFolder Is Nothing Then
        For i = 1 To UBound(FoldersArray, 1)
            Dim SubFolders As Outlook.Folders
            Set SubFolders = oFolder.Folders
            Set oFolder = SubFolders.item(FoldersArray(i))
            If oFolder Is Nothing Then
                Set GetFolderPath = Nothing
            End If
        Next
    End If
    'Return the oFolder
    Set GetFolderPath = oFolder
    Exit Function
        
GetFolderPath_Error:
    Set GetFolderPath = Nothing
    Exit Function
End Function

Use a shared folder (Exchange mailbox)

To access a shared folder in another user's Exchange server mailbox, you need to use GetSharedDefaultFolder to reference the mailbox, after resolving the address to the folder. If the shared folder was opened from a sharing invitation (or Open Other User's folder command), you may need to use the method in the next section.

You can use the mailbox owner's display name, alias, or email address when resolving the recipient.

  Dim NS As Outlook.NameSpace
  Dim objOwner As Outlook.Recipient
   
  Set NS = Application.GetNamespace("MAPI")
  Set objOwner = NS.CreateRecipient("maryc")
    objOwner.Resolve
       
 If objOwner.Resolved Then
   'MsgBox objOwner.Name
 Set newCalFolder = NS.GetSharedDefaultFolder(objOwner, olFolderCalendar)
 End If

To add an item to a folder in a shared mailbox, use Items.add:

Set objAppt = newCalFolder.Items.Add(olAppointmentItem)

Default Folder Names

The following are the default folder names to use when referencing Outlook's default folders.

Access a Folder Opened from a Sharing Invitation

In order to access a folder received from a folder Invitation (or using Open Other User's folder command), you need find the folder on the navigation pane. This will work with default folders opened using Open Other User's Folder command or any folder shared from a sharing invitation.

    Dim objPane As Outlook.NavigationPane
    Dim objModule As Outlook.CalendarModule
    
    Set objPane = Application.ActiveExplorer.NavigationPane
    Set objModule = objPane.Modules.GetNavigationModule(olModuleCalendar)
    
    With objModule.NavigationGroups
    
        For g = 1 To .Count
        Set objGroup = .Item(g)
    
            For i = 1 To objGroup.NavigationFolders.Count
            Set objNavFolder = objGroup.NavigationFolders.Item(i)
    
' need to use the calendar name as displayed in calendar list
            If objNavFolder = "Philip Poremsky - Test" Then
             Set CalFolder = objNavFolder.Folder
            End If
            Next
        Next
    End With