Get all users id with custom subscription(membership) level

get_all_users_id_with_custom_subscription_level.php

<?php

function get_all_users_id_by_membership_level_id() {
  
  $memberships = rcp_get_memberships( array(
    'status'    => 'active',
    'object_id' => 2 // Membership Levels ID
  ) );

  foreach ($memberships as $membership) {

    $customers[] = $membership->get_customer()->get_user_id();

  }

  $users_id = implode(',', $customers);

  return $users_id;
}