PowerShell function to invite an external user or list of external users to a SharePoint site

Add-SharePointSiteExternalUser.ps1

<#
.SYNOPSIS
    Invites an external user or list of external users to a SharePoint site.
.DESCRIPTION
    Invites an external user or list of external users to a SharePoint site. 
    External users can be granted View, Edit, or Owner permissions within 
    the site. A custom e-mail message can be included or the invitation email 
    can be suppressed.
.PARAMETER SiteURL
    URL string of the SharePoint site to which the user(s) will be invited.
.PARAMETER Crednetial
    A PowerShell Crednetial obejcet for the ShaerPoint site admin credentials. 
    Crednetials will be prompted if the paramaerter is ommited.
.PARAMETER User
    Either a string, list of strings, net.mail.mailaddress object, or list of 
    net.mail.mailaddress objects for the user or user to invte. Strings must 
    be valid email addresses.
.PARAMETER Permission
    Either View, Edit, or Owner. This permission level will be granted to the 
    invited user whtint the SharePoint site.
.PARAMETER Message
    Optional message to include with the e-mail invideation to the user. A 
    null or empty message will resutlt in the default invitation e-mail.
.PARAMETER SendNotificationEmail
    Switch to enable or suppress the e-mail invitaion tot he user. Default 
    is to send the message.
.EXAMPLE
    $SPCredentials = Get-Credential
    $SPSite = "https://contoso.sharepoint.com/sites/SharedSite"
    $User = "Bob.Testerton@fabrikam.com"
    $Message = "Hey Bob! Welcome to Contoso's Shared Site!"
    Add-SharePointSiteExternalUser -SiteURL $SPSite -Credential $SPCredentials -User $User -Permission View -Message $Message
.EXAMPLE
    $SPCredentials = Get-Credential
    $SPSite = "https://contoso.sharepoint.com/sites/SharedSite"
    $Users = "Bob.Testerton@fabrikam.com","Jill.Deverson@adatum.com"
    $Message = "Welcome to Contoso's Shared Site!"
    Add-SharePointSiteExternalUser -SiteURL $SPSite -Credential $SPCredentials -User $Users -Permission View -Message $Message
.EXAMPLE
    $SPCredentials = Get-Credential
    $SPSite = "https://contoso.sharepoint.com/sites/SharedSite"
    $ListFile = "C:\scripts\Lost_Of_Email_addresses.txt"
    $Message = "Welcome to Contoso's Shared Site!"
    Get-Content $ListFile | Add-SharePointSiteExternalUser -SiteURL $SPSite -Credential $SPCredentials -Permission View -SendNotificationEmail:$false
.INPUTS
    System.String. List of e-mail addresses.
    Net.Mail.Mailaddress. List of e-mail addresses.
.OUTPUTS
    System.Management.Automation.PSObject. Contains the SiteURL, User, Permission, 
    Success, and StatusMessage. StatusMessage may be null if successful.
.NOTES
    Requires the SharePoint Client Libraries. These can be installed either 
    with a SharePoint on-prem system or with the SharePoint Online Management Shell.

    https://www.microsoft.com/en-us/download/details.aspx?id=35588

    External Sharing must be turned on for both the Tenant and the Site Collection. 
    Depending on the sharing option configured, the user may also need to already 
    exist in tenant's Azure AD.

    https://support.office.com/en-us/article/Manage-external-sharing-for-your-SharePoint-Online-environment-c8a462eb-0723-4b0b-8d0a-70feafe4be85


    MIT License

    Copyright (c) 2016 Mark Kraus

    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:
    
    The above copyright notice and this permission notice shall be included in all
    copies or substantial portions of the Software.
    
    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
    SOFTWARE.
.LINK    
    https://www.microsoft.com/en-us/download/details.aspx?id=35588
.LINK
    https://gist.github.com/markekraus/6fd7218b8eac225cbbf4d7c31018331b#file-add-sharepointsiteexternaluser-ps1
.LINK
    https://support.office.com/en-us/article/Manage-external-sharing-for-your-SharePoint-Online-environment-c8a462eb-0723-4b0b-8d0a-70feafe4be85
#>
Function Add-SharePointSiteExternalUser {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory=$true)]
        [ValidateScript({[system.uri]::IsWellFormedUriString($_,[System.UriKind]::Absolute)})]
        [string]$SiteURL,
        [Parameter(Mandatory=$true)]
        [ValidateNotNull()]
        [System.Management.Automation.PSCredential]$Credential,
        [Parameter(Mandatory=$true,ValueFromPipeline=$True)]
        [net.mail.mailaddress[]]$User,        
        [Parameter(Mandatory=$true)]
        [ValidateSet("View","Edit","Owner")]
        [string]$Permission,
        [Parameter(Mandatory=$false)]
        [string]$Message = $null,
        [Parameter(Mandatory=$false)]
        [switch]$SendNotificationEmail = $true

    )
    begin{
        $StatusOK = $True
        Write-Verbose "Initializing SharePoint Client Libraries"
        try{
            $loadInfo1 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client")
            $loadInfo2 = [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime")
        }
        catch{
            Write-Error "Failed to load SharePoint Client Libraries."
            $StatusOK = $False
            break
        }
        Write-Verbose "Initializing SharePoint context object."
        try{
            $ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
            $SharePointCreds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Credential.UserName, $Credential.Password)
            $ctx.Credentials = $SharePointCreds
            $SharingManager = [Microsoft.SharePoint.Client.Sharing.WebSharingManager]
        }
        catch{
            Write-Error "Failed to initialize SharePoint context object. Ensure you have the correct permissions on the sharepoint site."
            $StatusOK = $False
            break
        }
        switch ($Permission){
                "View" {$SetPermission = [Microsoft.SharePoint.Client.Sharing.Role]::View}
                "Edit" {$SetPermission = [Microsoft.SharePoint.Client.Sharing.Role]::Edit}
                "Owner"{$SetPermission = [Microsoft.SharePoint.Client.Sharing.Role]::Owner}
        } 
    }
    process{
        if(!$StatusOK){return}
        $User | ForEach-Object {
            $CurUser = $_.Address.ToString()
            Write-Verbose "Granting '$CurUser' '$Permission' access to '$SiteURL'."
            $userList = New-Object "System.Collections.Generic.List``1[Microsoft.SharePoint.Client.Sharing.UserRoleAssignment]"
            $userRoleAssignment = New-Object Microsoft.SharePoint.Client.Sharing.UserRoleAssignment
            $userRoleAssignment.UserId = $CurUser
            $userRoleAssignment.Role = $SetPermission
            $userList.Add($userRoleAssignment)
            try{
                $res = $SharingManager::UpdateWebSharingInformation($ctx, $ctx.Web, $userList, $SendNotificationEmail, $message, $true, $true)
                $ctx.ExecuteQuery()
                $Success = $res.Status
                $StatusMessage = $res.message
            }
            catch{
                write-error "Error granting '$CurUser' '$Permission' access to '$SiteUR'."
                $Success = $False
                $StatusMessage = "Error granting '$CurUser' '$Permission' access to '$SiteUR'."
            }
            $ObjProperties = @{
                SiteURL = $SiteURL
                Permission = $Permission
                User = $CurUser
                Success = $Success
                StatusMessage = $StatusMessage
            }
            $OutObj = new-object psobject -Property $ObjProperties
            Write-Output $OutObj
        }
    }
}