I see many organizations with several more zeroes in their lawyer budget than I've got use the "notification via sign" route.
Usage matters. Having a gallery of "people having fun at our event" probably doesn't require a release. No evidence of it being litigated far enough to generate precedent at least. So you've got people using a dubious release methodology in circumstances where they probably don't need a release anyway.
In other areas there's at least vaguely the idea that "a sample of what we do" doesn't need a release. In one notable case a newspaper got away with using an unreleaseded photo of Joe Montana in advertising on those grounds.
But for most orgs winning in court is nearly as catastrophic as losing. Safer to make a lawsuit so clearly hopeless you can get a summary judgment rather than going to trial.
Anything with the faintest whiff of endorsement absolutely does require a release. At which point I'd want one all signed and legal.
Also: you don't say what your non-profit does. If any unwelcome conclusions could be drawn about someone due to their presence at your event (think AIDS charity maybe) you need to be hyper-careful.
But online advice is no substitute for a real lawyer.