$guestusers = Get-MgUser -Filter "UserType eq 'Guest'" -All -property signinactivity 
$output = @()

ForEach ($guestuser in $guestusers) {
    $output += New-Object -TypeName PSObject -Prop ([ordered]@{'Mail'=$guestuser.Mail;'LastSignInDateTime'=($guestuser | Select -ExpandProperty SignInActivity).LastSignInDateTime})
}

$output