need to set CustomAttribute based on mailboxGUID

Experts ... .rather basic PowerShell skills - I need a way to get GUID value from a set of mailboxes, and put that value into CustomAttribute1 for exchange. I've got this:

The problem is that the GUID inserted into CustomAttribute1 is the GUID from the first mailbox, so all get the same error.
Something here... ??? (set-mailbox $user -CustomAttribute1 "iXOS-EXLK:/{$($GUID.Guid)}" ???)
 ####################################################################################################
 #
 # Script for retrieving members of  group, extract  GUID and set
 # CustomAttribute1 generated from iXOS-EXLK:/+GUID
 #
 #
 ######################################################################################################

 # Get users from AD-Group
 # Alternatively - Group can be entered by user
 #
 # $AdGroup = Read-Host "Enter group name"
 # $users = Get-ADGroupMember $AdGroup | select -ExpandProperty name
 #
 # Selecting Users from Group Test_MBX_Permission

 $users = Get-ADGroupMember Test_MBX_Permission | Select-Object -ExpandProperty name
 foreach ($user in $users)
{
 # Extract Mailbox GUID and pipe to variable $GUID
$GUID = Get-Mailbox $user | select GUID
}
# Set CustomAttribute1 based on iXOX-EXLK:/ + Users unique MailboxGUID
 foreach ($user in $Users)
 {
 $customAttrib1 = Get-Mailbox -Identity $user -CustomAttribute1 -ErrorAction SilentlyContinue | select CustomAttribute1
 if($customAttrib1)
 {
 #If customattribute1 have value - do nothing
 }
 else
 {
#set-mailbox with value CustomATtribute1
 set-mailbox $user -CustomAttribute1 "iXOS-EXLK:/{$($GUID.Guid)}"
 }

 }