WP-Members allows for customizing the registration form validation. This feature can be used to restrict registration based on the user’s information such as email address, IP address, or some other data point. This article covers how to do that and includes some basic examples.
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.