Live tweet your event
Start a Twitter conversation (with the correct hashtag, of course) related to your event. Ask questions. Create polls. Have people join the thread and write their insights on the event.
And there are three big names to boot - Facebook Stories, Instagram Stories (the latter two of which can cross-post), and Snapchat. Stories make great platforms to share quick photos and videos about what's happening to your event, right now, and in real-time.
Collaborate with an influencer
And it's way much better if said influencer is the guest speaker or event host. This attracts more audience, giving your brand authority.
If you find live-tweeting or posting a story too cumbersome, you can also enable a live Q&A session during the event. You could enable live chat where participants can type questions in a chatbox as they wish, or you can gather 5-10 questions from people and announce them live.