Private/ListPrivate/Get-GSGroupMemberListPrivate.ps1

function Get-GSGroupMemberListPrivate {
    [cmdletbinding()]
    Param
    (
        [parameter(Mandatory = $true,Position = 0,ValueFromPipeline = $true,ValueFromPipelineByPropertyName = $true)]
        [Alias('GroupEmail','Group','Email')]
        [String[]]
        $Identity,
        [parameter(Mandatory = $false)]
        [ValidateSet("Owner","Manager","Member")]
        [String[]]
        $Roles,
        [parameter(Mandatory = $false)]
        [ValidateRange(1,200)]
        [Alias('MaxResults')]
        [Int]
        $PageSize = "200"
    )
    Begin {
        $serviceParams = @{
            Scope       = 'https://www.googleapis.com/auth/admin.directory.group'
            ServiceType = 'Google.Apis.Admin.Directory.directory_v1.DirectoryService'
        }
        $service = New-GoogleService @serviceParams
    }
    Process {
        foreach ($Id in $Identity) {
            try {
                if ($Id -notlike "*@*.*") {
                    $Id = "$($Id)@$($Script:PSGSuite.Domain)"
                }
                $request = $service.Members.List($Id)
                if ($PageSize) {
                    $request.MaxResults = $PageSize
                }
                if ($Roles) {
                    Write-Verbose "Getting all members of group '$Id' in the following role(s): $($Roles -join ',')"
                    $request.Roles = "$($Roles -join ',')"
                }
                else {
                    Write-Verbose "Getting all members of group '$Id'"
                }
                [int]$i = 1
                do {
                    $result = $request.Execute()
                    $result.MembersValue | Add-Member -MemberType NoteProperty -Name 'Group' -Value $Id -PassThru  | Add-Member -MemberType ScriptMethod -Name ToString -Value {$this.Email} -PassThru -Force
                    $request.PageToken = $result.NextPageToken
                    [int]$retrieved = ($i + $result.MembersValue.Count) - 1
                    Write-Verbose "Retrieved $retrieved members..."
                    [int]$i = $i + $result.MembersValue.Count
                }
                until (!$result.NextPageToken)
            }
            catch {
                if ($ErrorActionPreference -eq 'Stop') {
                    $PSCmdlet.ThrowTerminatingError($_)
                }
                else {
                    Write-Error $_
                }
            }
        }
    }
}