For referrals, it could be an idea to gamify the experience. Create a leaderboard with some goodies like promotional merchandise, VIP access to a backstage area or “meet the speakers” area and use that as a way to drive increased signup rates. Tonnes of tech companies use referral marketing in the same way for their products, like Uber or The Hustle (which is a pretty awesome newsletter).
For general word of mouth, it works to be provocative or have a highly sought after speaker attending, though I appreciate this won’t be possible for some organisers. Another way to do it could be to facilitate your attendee’s own conversations. Set up a dedicated networking space, and encourage attendees to bring clients or friends along. Can companies receive Continuing Professional Development (CPD) for your events, or are they eligible for free funding from your L&D department? It’s always good to have some templates which make it easier for people to attend.
The biggest growing events that I’ve seen have had some viral or undeserved element, like Brighton SEO which you’ve written about earlier. They grew from 20 people attending to thousands through great content, speakers and parties - that’s always one where the real word of mouth might play in!