Assign default membership to an user when registers

code.php

<?php

add_action( 'user_register', 'add_default_membership', 10, 1 );
function add_default_membership( $user_id ) {
	
	// Give the default membership ID here
	$memberships = array( 1601 );
	
	$member = MS_Factory::load( 'MS_Model_Member', $user_id );
	
	// Add new memberships
	foreach ( $memberships as $membership_id ) {
		$subscription = $member->add_membership( $membership_id );
	
	}
	
	if ( $member->has_membership() ) {
		$member->is_member = true;
	} else {
		$member->is_member = false;
	}
	$member->save();

}