// log details of accepted group invitation
add_action('peepso_action_group_user_invitation_accept', function(PeepSoGroupUser $PeepSoGroupUser) {
    $msg = 'User ' . get_current_user_id() . ' accepted the invitation from {$PeepSoGroupUser->$invited_by_id} to join group {$PeepSoGroupUser->$group_id}';
    error_log($msg);
}, 10, 1);