I'm not sure how to prompt a trial membership off an event, however you could email a Promo Code to all of your Event Registrants. Basically any Registrant that never joined your club will have a Status or Access Level of Event Registrant OR Active NonRevenue. So you can use those status designations to populate a Newsletter Message, and include a Promo Code for a Free Trial or discounted Membership Plan.
We want to start tracking registrations and offering trial memberships prompted by particular events. We were thinking of using promotion codes for this however I can't find anything in the KB or Help menu on their use. Can you offer some research and "getting started" suggestions?