@Brady's answer is correct because WP-Members does use WP's function for inserting a new user, so that action does get called. And if you want flexibility and you are only using the new user's ID for all of it, that's a good approach.
But a secondary approach would be to use WP-Members' wpmem_post_register_data
action. That action passes all of the user's registration data an array keyed by the field meta keys and it includes ID.
add_action( 'wpmem_post_register_data', 'my_reg_hook' );
function my_reg_hook( $fields ) {
/*
* ID is available as $fields['ID']
* Other fields are $fields['the_fields_meta_key']
*
* do your stuff here
*/
return;
}
The documentation for the action has a list of fields that will be included in the array in addition to any custom fields that are in there by meta key:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_post_register_data/