Insert typical, I’m not a lawyer text… okay, good.
My gut says you will need a release from the location and, at least, the main people featured in the event (ie: fighters, referees, coaches, and/or anyone featured prominently). Won’t likely need releases from the audience unless they are featured prominently (edit around those bits to make them less prominent). May also need a property release from the fight location.
Think in terms of big sporting events like NFL. Everyone signs a release: coaches, staff, players, refs, even press. Basically, if anyone CAN be on camera AND they can get a release signed, they do. Its a pile of paperwork, but its worth it.
If you need to get releases, make sure to get the exact date or date ranges (time is helpful, but not necessary) of the event, get them to sign and offer them $1. If you want a good release, do a search in Google for “Getty model release” (and Getty property release)and click the PDF to download. Make the appropriate changes for your business and run it by a lawyer to be sure. Both releases need a witness signature (can be any third party, but NOT you or the other signer) to be valid and at least $1 consideration. In most cases, I’ll get away with buying someone lunch as payment and I attach the receipt to the release and it’s a legit business expense.
Jonathan Ziegler
https://www.electrictiger.com/
520-360-8293