Definitely agree @KtMcPhee that using influencers on twitter has got less effective - i’d like to know if people are still having success here? I think the influencer party is moving to Instagram, but i’m just less clued up about that platform to get properly down to grips with it, and it also seems more suitable for b2c than b2b.
Having said that, a great free promo tactic i’ve used in the past is not just asking influencers to attend/promote, but getting your headline acts/speakers to promote on twitter/fb as well as create content like blog posts. It’s in their interest to have a full house so they often don’t mind being ask. If you think a full blog post is a bit much get them to give a short answer to a questions such as "why are you excited about my-event, then collate all the answers into a blog post and push that out there on social as “x reasons to attend” People would much rather read that kind of content than a tweet that just says “get tickets…” as it communicates the value of the event far better.