Answered in PM's but placing it here since its pretty important to know:

Aiden and Serena only actively recruit from the White Harbor area, since they don't have the resources to advertise elsewhere, both in terms of funds and manpower. However, Winter's Blade has likely made the rounds in guild circles due to the interesting circumstances around it and Aiden's own accomplishments and the Mage's Council keeps a public registry of all active guilds present in most major cities, so finding out about the guild and where it is isn't that difficult.