Can I use pictures of people for my marketing materials without permission?

In California, you definitely need a very well-written release form for this and give notice in very prominent lettering on all materials, as well as at the door. In the Los Angeles area, particularly, many people who may attend such a workshop may be actors or models, and may have agents or belong to a performer's union. You cannot just snap a photo of such a person and use it in your marketing materials. You must arrange for this in advance, sign a contract for their services, and pay them their rate. Therefore, particularly in California, you are best off to separate your workshop from your marketing, and either hold a special staged event for taking photos, or use great caution to take only photos of those who have agreed in writing to be on your marketing materials.

Yes, you must get a release to show faces or use names or quotes in marketing materials. There is an exception for general public shots in public places as long as you don't imply that the people in the shots are unlawful, unattractive, engaged in criminal or socially unacceptable practices or anything else that would cast a shadow of their name, person, or reputation.