Index: src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
===================================================================
--- src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php	(revision 39153)
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php	(working copy)
@@ -434,6 +434,8 @@
 			if ( is_wp_error( $user_id ) ) {
 				return $user_id;
 			}
+
+			add_user_to_blog( get_site()->id, $user_id, '' );
 		} else {
 			$user_id = wp_insert_user( $user );
 
@@ -558,6 +560,10 @@
 
 		$user = get_user_by( 'id', $id );
 
+		if ( is_multisite() && ! is_user_member_of_blog( $id ) ) {
+			add_user_to_blog( get_current_blog_id(), $id, '' );
+		}
+
 		if ( ! empty( $request['roles'] ) ) {
 			array_map( array( $user, 'add_role' ), $request['roles'] );
 		}