View all external users who have been granted access

Andrew van Renen, I am not sure if this will help. But you can use this script to get permission details for selected document library. HTH. 

Param(
[Parameter(Position=0, Mandatory=$true, ValueFromPipeline=$true)]
[string] $Username,
[Parameter(Position=1, Mandatory=$true, ValueFromPipeline=$true)]
[string] $Password,
[Parameter(Position=2, Mandatory=$true, ValueFromPipeline=$true)]
[string] $Site,
[Parameter(Position=3, Mandatory=$true, ValueFromPipeline=$true)]
[string] $Library
)

Function Main
{
#Add references to SharePoint client assemblies and authenticate to Office 365 site – required for CSOM
Add-Type -Path ([System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client").location)
Add-Type -Path ([System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.runtime").location)

#Bind to site collection
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($Site)
$creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Username,$(convertto-securestring $Password -asplaintext -force))
$ctx.Credentials = $creds

#Retrieve list
$list = $ctx.Web.Lists.GetByTitle($Library)
$ctx.Load($list)
$ctx.ExecuteQuery()

#Get User's permission in the list
$listRoleAssignments = $List.RoleAssignments
$ctx.Load($listRoleAssignments)
$ctx.ExecuteQuery()

$rec=@()
Foreach($assignment in $listRoleAssignments)
{
$userPermission=$assignment.RoleDefinitionBindings
$memberName=$assignment.Member
$ctx.Load($memberName)
$ctx.Load($userPermission)
$ctx.ExecuteQuery()
$extUserDetails = "" | Select "UserName","Loginname", "Permission"
$userLoginName=$membername.LoginName.ToString()

if($userLoginName -like '*#ext#*')
{
$extUserDetails.Loginname =$userLoginName
$extUserDetails.UserName = $memberName.Title
$extUserDetails.Permission = $UserPermission.name
$rec+= $extUserDetails
}
}
#List out for all External Username with Permission
Write-Host "Total number of External users :" $rec.Count
Write-Host "All External users with Permission details.."
Write-Host "-----------------------------------------------"
$rec
}

Main-Username <Name> -Password <Password>-Site <SiteURL> -Library <document library name>