I haven't come across this particular problem, and you might get more info.
in an Outlook group,  but...

A user ought to be able to modify all of their own contacts with a bit of
Outlook (or Word) VBA. Depending on how many people you have to update, that
might or might not be a manageable solution. Again, you might be better off
asking how to do this in an Outlook group as they probably know more about
the pitfalls, but the following code worked in a simple case here where all
the contacts are in the default contacts folder (that isn't necessarily the
case). In order to force an update, it adds and removes a user-defined
property to/from every contact - in principle you ought to check for the
name of the user defined field before trying to add it. To use it from the
Word VBA editor you need to use Tools|References to add the Microsoft ??
Outlook Object Library, where 11 is your Outlook version.

Sub touch_all_contact_records()

Dim oOutlookApp As Outlook.Application
Dim oContactFolder As Outlook.MAPIFolder
Dim oContactItem As Outlook.ContactItem
Dim oUserProperty As UserProperty
Dim oONS As Outlook.NameSpace

' If you are running this from Outlook, use this...
'Set oOutlookApp = Application
' Otherwise, use this...
Set oOutlookApp = CreateObject("Outlook.Application")
Set oONS = oOutlookApp.GetNamespace("MAPI")
Set oContactFolder = oONS.GetDefaultFolder(olFolderContacts)

' Add all the contact items in the default contacts folder to the list

For Each oContactItem In oContactFolder.Items
 If Left(oContactItem.MessageClass, 11) = "IPM.Contact" Then
   Set oUserProperty =
oContactItem.UserProperties.Add(Name:="u432765874635", Type:=olText,
addtofolderfields:=False)
   oContactItem.Save
   oUserProperty.Delete
   Set oUserProperty = Nothing
   oContactItem.Save
   oContactItem.Close olSave
 End If
Next

Set oContactFolder = Nothing
Set oONS = Nothing
Set oOutlookApp = Nothing

End Sub

Peter Jamieson