For your audiance you will need to provide assurances- maybe make the standard terms (maybe even the amounts?) public. Give each company a spot on the forum- as well as the magazine- to give feedback to potential customers so there are no stealth members. Continue to offer testing to all companies with no priorities, and publish the list of who was invited to take part. If a company does not respond or refuses let the audiance know and then, invite them again the next year or test them anyway with a demo from a shop.