Below is some of the legal prep that you may want to do in preparation for host an event:

Photo Release For Attendees: If your event is going to have a photographer or videographer, or even if you’re planning to take photos with your phone, you want to make sure you have permission to use those event photos in your future marketing. If your attendees' faces are shown, you must have their permission to use those images. Make sure you ask attendees to sign a photo or model release so that you’re able to use the images captured at your event without any restrictions.

Photo License: You can also include a photo license in the agreement, which allows you, the organizer, to use photos that your attendees post on their own social media, in your marketing and communications. This is great if your attendees will be posting their experience and you’d like to use their images, too.
Participant Release and Indemnification Provision: It's a good idea to have attendees sign a release (preferably drafted by an attorney to ensure compliance with state and local laws) for your event. Additionally, if you’re going to be serving alcohol or having any kind of physical activity (e.g. yoga class or dance parties). This release can help protect you if an attendee gets injured especially, if you weren’t actively putting attendees in harm’s way.
Copyright notice: Include this in all of your attendee agreements to note that you own all the copyrights to your event content. This forewarns people about what they can and can't do with the content. Can they share it with a friend or is it just for their personal use? Can they put it on the internet? Can they resell it? Be clear and make sure your attendees understand.
Sponsorship Agreements: If you have brands contributing monetarily and/or with goods/services, then you’ll need a contract to make sure that the parties are in agreement about what is being required for the sponsorship dollars and/or items.
Speaker Agreements: If you have people speaking at your event, whether compensated or not, then make sure there is an agreement reflecting what is to be expected between the parties. For instance, if you require the speaker to provide materials, presentations, or promotions by a certain date, then put it in writing so the expectation is really clear.