Hello,
I would recommend creating a completely separate tenant for this type of event. You don't even need a domain and could use "newco.onmicrosoft.com" try conduct the training for Azure DevOps.
If they have their own devices, they could easily login through a browser.
If they don't have their own devices, configure your devices to have a local account that they can log into, or domain the machines with "newco.onmicrosoft.com" and they can sign in as themselves.
Hopefully this gives you some ideas.