WP-Members allows for customizing to restrict registration based on the user’s information. Often people ask how they might restrict registration based on the user’s email address, IP address, or some other data point. This article covers some different approaches to that.
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.