PowerShell script which processes an Azure AD B2B invitation CSV file (augmented with an `SPGroups` fields) and grants access permissions to SharePoint Online accordingly.

Add-B2BUsersToSite.ps1

[CmdletBinding()]
Param(
    [Parameter(Mandatory=$True)]
    [string]$CSVFilename,
    
    [Parameter(Mandatory=$True)]
    [string]$SiteUrl,
    
    [Parameter()]
    $Credentials
)

function AddB2BUsersToSite() {
    # Check that the CSV file exists
    if (-not (Test-Path $CSVFilename)) {
        Write-Error "Could not find CSV-file. Please check the path and try again."
        return
    }

    # Prompt for credentials (if not passed in as parameter)
    if (-not $Credentials) {
        Write-Verbose "Credentials not supplied, prompting for them."
        $Credentials = Get-Credential -Message "Login to $siteUrl"
    }

    # Connect to SPOnline
    try {
        Write-Debug "Connecting to SharePoint"
        Connect-SPOnline -Url $SiteUrl -Credentials $Credentials
        $web = Get-SPOWeb
    } catch {
        Write-Error "Could not connect to $siteUrl. Please check the provided url and credentials and try again."
        return
    }

    # Open the CSV-file
    Write-Debug "Opening CSV file"
    $csv = Import-Csv -Delimiter "," $CSVFilename

    # Loop through all users
    $csv | ForEach-Object {
        $row = $_
        $loginName = $row.Email
        $spgroups = $row.SPGroups

        # Add the user to the hidden user information list
        # This will also make the user appear in the PeoplePicker dialog
        Write-Output "Adding $loginName to site"               
        Write-Debug  "Ensuring that user is present"
        $user = $web.EnsureUser($loginName)
        $web.Context.ExecuteQuery()

        # Add the user to the specified groups
        Write-Verbose "Adding user to groups"
        if ($spgroups) {
            $spgroups -split '\+' | ForEach-Object {                
                $groupName = $_.Trim()
                Write-Verbose "- Adding user to group: $groupName"
                Add-SPOUserToGroup -identity $groupName -LoginName $loginName
            }
        }
    }
}

# Main
try {
    AddB2BUsersToSite
} finally {
    try {Disconnect-SPOnline -ErrorAction SilentlyContinue } catch {}
}