This code snippet adds all approved users whose Ultimate Member profiles are public to the sitemap.

um_sitemaps_users.php

<?php

/**
 * Add all approved users whose Ultimate Member profiles are public to the sitemap.
 * Add this code to the file functions.php in the active theme directory.
 */ 

add_filter( 'wp_sitemaps_max_urls', 'um_sitemaps_users_max_urls', 10, 2 );
add_filter( 'wp_sitemaps_users_pre_max_num_pages', 'um_sitemaps_users_max_num_pages', 10, 1 );
add_filter( 'wp_sitemaps_users_pre_url_list', 'um_sitemaps_users_get_url_list', 10, 2 );

/**
 * Filters the maximum number of URLs displayed on a users sitemap.
 *
 * @see https://developer.wordpress.org/reference/hooks/wp_sitemaps_max_urls/
 *
 * @param  int    $max_urls    The maximum number of URLs included in a sitemap. Default 2000. Change to 100 for users.
 * @param  string $object_type Object type for sitemap to be filtered (e.g. 'post', 'term', 'user').
 * @return int
 */
function um_sitemaps_users_max_urls( $max_urls, $object_type ) {
	if ( 'user' === $object_type ) {
		$max_urls = 100;
	}
	return $max_urls;
}

/**
 * Filters the max number of pages for a user sitemap before it is generated.
 * Returning a non-null value will effectively short-circuit the generation.
 *
 * @see https://developer.wordpress.org/reference/hooks/wp_sitemaps_users_pre_max_num_pages/
 *
 * @param  int|null $max_num_pages The maximum number of pages. Default null.
 * @return int
 */
function um_sitemaps_users_max_num_pages( $max_num_pages ) {

	if ( get_option( 'blog_public' ) ) {

		$args = array(
			'fields'     => 'ids',
			'meta_query' => array(
				array(
					'key'     => 'account_status',
					'value'   => 'approved',
					'compare' => '=',
				),
			),
		);

		$query       = new WP_User_Query( $args );
		$total_users = $query->get_total();

		if ( 0 < $total_users ) {
			$max_num_pages = (int) ceil( $total_users / wp_sitemaps_get_max_urls( 'user' ) );
		}
	}

	return $max_num_pages;
}


/**
 * Filters the users URL list before it is generated.
 * Returning a non-null value will effectively short-circuit the generation.
 *
 * @see https://developer.wordpress.org/reference/hooks/wp_sitemaps_users_pre_url_list/
 *
 * @param  array|null $url_list The URL list. Default null.
 * @param  int        $page_num Page of results.
 * @return array
 */
function um_sitemaps_users_get_url_list( $url_list, $page_num ) {

	if ( get_option( 'blog_public' ) ) {

		$args = array(
			'fields'     => 'ids',
			'number'     => wp_sitemaps_get_max_urls( 'user' ),
			'paged'      => $page_num,
			'meta_query' => array(
				array(
					'key'     => 'account_status',
					'value'   => 'approved',
					'compare' => '=',
				),
			),
		);

		$query        = new WP_User_Query( $args );
		$users        = $query->get_results();
		$url_list_new = array();

		foreach ( $users as $user_id ) {

			/**
			 * Skip users whose profiles can not be indexed by searching engines.
			 * Settings that influence profile indexing by the priority:
			 *  "Search engine visibility" in [wp-admin > Settings > Reading]
			 *  "Profile Privacy" in [Account > Privacy]
			 *  "Avoid indexing my profile by search engines in [Account > Privacy]
			 *  "Avoid indexing profile by search engines" in [wp-admin > Ultimate Member > User Roles > Edit Role]
			 *  "Avoid indexing profile by search engines" in [wp-admin > Ultimate Member > Settings > General > Users]
			 */
			if ( UM()->user()->is_profile_noindex( $user_id ) ) {
				continue;
			}

			$sitemap_entry = array(
				'loc' => um_user_profile_url( $user_id ),
			);

			$url_list_new[] = apply_filters( 'um_wp_sitemaps_users_entry', $sitemap_entry, $user_id );
		}

		if ( 0 < count( $url_list_new ) ) {
			$url_list = $url_list_new;
		}
	}

	return $url_list;
}

This gist is a part of the article How to add profiles to sitemap.

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/

Example: