First pass at a custom `newbloguser` handler.

includes/actions.php

@@ -63,3 +63,56 @@ function wp_join_page_maybe_do_request() {
 	wp_safe_redirect( $url );
 	die;
 }
+
+/**
+ * Add a new user to a blog by visiting /newbloguser/username/.
+ *
+ * This will only work when the user's details are saved as an option
+ * keyed as 'new_user_x', where 'x' is the username of the user to be
+ * added, as when a user is invited through the regular WP Add User interface.
+ *
+ * @since 0.1.0
+ */
+function wp_join_page_maybe_add_existing_user_to_blog() {
+
+	// "newbloguser" is a hard-coded URL segment, for now
+	if ( false === strpos( $_SERVER[ 'REQUEST_URI' ], '/newbloguser/' ) ) {
+		return;
+	}
+
+	// Break apart the request
+	$parts = explode( '/', $_SERVER[ 'REQUEST_URI' ] );
+	$key   = array_pop( $parts );
+
+	// Get username from URL
+	if ( empty( $key ) ) {
+		$key = array_pop( $parts );
+	}
+
+	// Clean-up
+	$option_key = "new_user_{$key}";
+	$details    = get_option( $option_key );
+	if ( ! empty( $details ) ) {
+		delete_option( $option_key );
+	}
+
+	// Get blog name
+	$blog_name = get_option( 'blogname' );
+	$home_url  = home_url();
+
+	// Failure
+	if ( empty( $details ) || is_wp_error( add_existing_user_to_blog( $details ) ) ) {
+		wp_die(
+			sprintf( __( 'An error occurred adding you to this site. Return to <a href="%s">%s</a>.'), $home_url, $home_url ),
+			sprintf( __( '%s &rsaquo; Error' ), $blog_name ),
+			array( 'response' => 500 )
+		);
+	}
+
+	// Success
+	wp_die(
+		sprintf( __( 'You have been added to this site. Now you can <a href="%s">log in</a> using your username & password.' ), wp_login_url( $home_url ) ),
+		sprintf( __( '%s &rsaquo; Success' ), $blog_name ),
+		array( 'response' => 200 )
+	);
+}

includes/dictionary.php

@@ -25,8 +25,8 @@ function wp_join_page_get_string( $key = '' ) {
 		$dictionary = array(
 
 			// Errors
-			'err_username' => esc_html__( 'This username is already in use. Do you already have an account?', 'wp-join-page' ),
-			'err_email'    => esc_html__( 'This email address is already registered. Do you already have an account?', 'wp-join-page' ),
+			'err_username' => esc_html__( 'That username is already in use. Do you already have an account?', 'wp-join-page' ),
+			'err_email'    => esc_html__( 'That email address is already registered. Do you already have an account?', 'wp-join-page' ),
 			'err_unknown'  => esc_html__( 'An unknown error occurred. Please try again with a more unique username.', 'wp-join-page' ),
 
 			// Success

includes/hooks.php

@@ -15,6 +15,9 @@ remove_action( 'wpmu_activate_user', 'wpmu_welcome_user_notification', 10, 3 );
 remove_action( 'after_signup_user',  'wpmu_signup_user_notification',  10, 4 );
 remove_action( 'init',               'maybe_add_existing_user_to_blog'       );
 
+// Custom user invitation acceptance action
+add_action( 'init', 'wp_join_page_maybe_add_existing_user_to_blog' );
+
 // Enqueue assets
 add_action( 'wp_enqueue_scripts', 'wp_join_page_enqueue_scripts' );