This article is a followup to “Restrict registration to specific email domains, IPs, or other data” that adds an element for moderated registration. In this case, moderated registration is used and registrants with emails accounts from whitelisted domains are automatically approved, leaving all others to be approved by an administrator.
It should be noted that this only applies to WP-Members registration. It has become more common to integrate WP-Members with other plugins that also have registration, such as WooCommerce, BuddyPress, bbPress, etc. If you are using one of these plugins for registration, that’s fine – but you need to then look into their process for custom validation.