$emailed = array('firstname.lastname@example.org', 'email@example.com');
$message = count($emailed) > 10
? implode(array_slice($emailed, 0, 10), ', ') . 'and ' . count($emailed) . ' others were sent an invite!'
: (count($emailed) > 2)
? 'Invites sent to: ' . implode($emailed, ', ') . '.'
: (count($emailed) == 2)
? implode($emailed, ' and ') . ' were invited'
: $emailed . ' was invited';