// log details of sent group invitation
add_action('peepso_action_group_user_invitation_send', function(PeepSoGroupUser $PeepSoGroupUser) {
    $msg = 'User ' . get_current_user_id() . ' invited user {$PeepSoGroupUser->$user_id} to group {$PeepSoGroupUser->$group_id}';
    error_log($msg);    error_log($msg);
}, 10, 1);