Assign role based on WPMU membership

code.php

<?php

add_action( 'ms_model_relationship_create_ms_relationship_before', 'ms_controller_member_assign_memberships_done_cb', 99, 4 );
function ms_controller_member_assign_memberships_done_cb( $membership_id, $user_id, $gateway_id, $move_from_id ) {
	$user = new WP_User( $user_id );
	switch( $membership_id ){
		case 1343:
			$user->set_role( 'editor' );
			break;
		
		case 1345:
			$user->set_role( 'author' );
			break;
		
		case 1348:
			$user->set_role( 'administrator' );
			break;
	}
}