Here are some possible functional roles your partners can fill to support your development role:
Project Manager: Ultimately responsible for timelines, budgets, and delivery of projects / sprints and product delivery.
Business Analysts: Collect customer requirements for the software. This role usually requires subject matter and market expertise or access to such expertise. They ultimately write the functional specs that your technical specs will be derived from.
Product Manager: This role compliments the marketing efforts by ensuring that first and all future iterations of the product are meeting market and customer needs. They have full responsibility for bridging that gap between functional and technical.
I think what you mentioned about also using them to write documentation and test is a great idea, as it will help them understand the development time and effort required from your job, and allow them to speak intelligently to the more technical aspects of the product, even if they are not techies themselves.
They could also play a big role in helping construct the user experience, by storyboarding and wire-framing.