Tips from the Trademark Insider: Codify All Licenses in Writing.

Whenever you or your company allows others - such as members, chapters, affiliated entities, or endorsed vendors – to use you or your company’s trademark(s), name, logos, copyrighted works, or other intellectual property, put the terms and conditions of the license in writing. While oral or implied non-exclusive licenses can exist, they can be difficult to interpret, difficult to enforce, limiting in nature, and otherwise problematic for you or your company.

The Trademark Company