Define a list of allowed users which can access the OptinMonster admin pages.

om-allowed-users.php

<?php

// Comma-separated list of WordPress user ids allowed access to the OptinMonster pages.
define( 'ALLOWED_OM_USERS', '1,2,5' );

// The new capability defined for viewing OM pages.
// function for_om_get_allowed_cap() {
// 	return 'optinmonster_allowed';
// }
// add_filter( 'optin_monster_api_menu_cap', 'for_om_get_allowed_cap' );

// // Checking given user against the list of allowed users.
// function for_om_is_user_in_allowed_list( $user_id ) {
// 	if ( ! defined( 'ALLOWED_OM_USERS' ) ) {
// 		return $user_id;
// 	}

// 	$allowed = array_map( 'absint', explode( ',', ALLOWED_OM_USERS ) );

// 	return in_array( absint( $user_id ), $allowed, true );
// }

// // Determines if given user has the new OM allowed capability.
// function om_check_if_user_allowed_access( $caps, $cap, $user_id ) {
// 	if ( for_om_get_allowed_cap() === $cap && ! for_om_is_user_in_allowed_list( $user_id ) ) {

// 		// Search for and remove the optinmonster_allowed capability.
// 		$key = array_search( for_om_get_allowed_cap(), $caps );
// 		if ( false !== $key ) {
// 			unset( $caps[ $key ] );
// 		}

// 		// Handles for multisite super admins, which have all caps by definition,
// 		// unless specifically denied.
// 		$caps[] = 'do_not_allow';
// 	}

// 	return $caps;
// }
// add_filter( 'map_meta_cap', 'om_check_if_user_allowed_access', 10, 3 );