My husband is an investigator for patent and trademark lawyers. Sometimes it's not about giving you express consent but their common law previous use, if they can demonstrate that. He investigates both ends of the conundrum. Clients want him to find out what someone with a similar mark/product/service is using it for (if at all) if they themselves want to register it or, if they want to take action against someone who is using it if they believe it breaches their rights to use it. The link here is to one of the firms that instruct him. It's not a recommendation because he works for lots. It's just an example. A look around most patent-law firm sites will give you insight and examples
http://www.ancienthume.com/why-register-trade-marks/
He gives you one last tip (and none of my post is legal advice, I hasten to add, it's just his experience over the years). It's better to make the approach before registration than be sued further down the line (which the link kind of covers). I'll shut up now.