Creating user relationships

WordPress does provide you with the ability to save meta data for users, so you could just store the agent’s user id in a meta field of his subscribers, like update_user_meta( $user_id, "agent", $agent_id ).

You can then query those users with the WP_User_Query’s meta_query, e.g.

$agents_subscribers = new WP_User_Query( array(
    'meta_key' => 'agent',
    'meta_value' => $agent_id
) );