“Customers who choose to engage with their brands become loyal brand ambassadors, and the work they create is far more credible than anything we could say about ourselves.” – “The six laws of collaborative branding” by Jason Baer (page 7).
By allowing customers to interact with the brand, they have more of an interest in the brand and they will be more likely to recommend or mention the brand to an acquaintance. I think “ambassador” is a good word here. The customer does not define the brand – we still need the company to do that. But customers can be ambassadors for the brand and extend the reach of the brand beyond what it might other have.