add_shortcode( 'groups_join_user', 'my_groups_join_user' );
function my_groups_join_user( $atts, $content = null ) {
    $nonce_action = 'groups_action';
    $nonce        = 'nonce_join';
    $output       = "";

    $options = shortcode_atts(
            array(
                    'group'             => '',
                    'user_id'             => '',
                    'display_message'   => true,
                    'display_is_member' => false,
                    'submit_text'       => __( 'Join the %s group', GROUPS_PLUGIN_DOMAIN )
            ),
            $atts
            );
    extract( $options );

    if ( $display_message === 'false' ) {
        $display_message = false;
    }
    if ( $display_is_member === 'true' ) {
        $display_is_member = true;
    }

    $group = trim( $options['group'] );
    $current_group = Groups_Group::read( $group );
    if ( !$current_group ) {
        $current_group = Groups_Group::read_by_name( $group );
    }
    if ( $current_group ) {
        if ( $user_id !== '' ) {
            $submitted     = false;
            $invalid_nonce = false;
            if ( !empty( $_POST['groups_action'] ) && $_POST['groups_action'] == 'join' ) {
                $submitted = true;
                if ( !wp_verify_nonce( $_POST[$nonce], $nonce_action ) ) {
                    $invalid_nonce = true;
                }
            }
            if ( $submitted && !$invalid_nonce ) {
                // add user to group
                if ( isset( $_POST['group_id'] ) ) {
                    $join_group = Groups_Group::read( $_POST['group_id'] );
                    Groups_User_Group::create(
                            array(
                                    'group_id' => $join_group->group_id,
                                    'user_id' => $user_id
                            )
                            );
                }
            }
            if ( !Groups_User_Group::read( $user_id, $current_group->group_id ) ) {
                $submit_text = sprintf( $options['submit_text'], wp_filter_nohtml_kses( $current_group->name ) );
                $output .= '<div class="groups-join">';
                $output .= '<form action="#" method="post">';
                $output .= '<input type="hidden" name="groups_action" value="join" />';
                $output .= '<input type="hidden" name="group_id" value="' . esc_attr( $current_group->group_id ) . '" />';
                $output .= '<input type="submit" value="' . $submit_text . '" />';
                $output .=  wp_nonce_field( $nonce_action, $nonce, true, false );
                $output .= '</form>';
                $output .= '</div>';
            } else if ( $display_message ) {
                if ( $submitted && !$invalid_nonce && isset( $join_group ) && $join_group->group_id === $current_group->group_id ) {
                    $output .= '<div class="groups-join joined">';
                    $output .= sprintf( __( 'The user has joined the %s group.', GROUPS_PLUGIN_DOMAIN ), wp_filter_nohtml_kses( $join_group->name ) );
                    $output .= '</div>';
                }
                else if ( $display_is_member && isset( $current_group ) && $current_group !== false ) {
                    $output .= '<div class="groups-join member">';
                    $output .= sprintf( __( 'The user is a member of the %s group.', GROUPS_PLUGIN_DOMAIN ), wp_filter_nohtml_kses( $current_group->name ) );
                    $output .= '</div>';
                }
            }
        }
    }
    return $output;
}