First, you must determine who will be coming to your workshops--people who sing in a "band" or people who just love to sing in a session? Then you must determine how long will the workshop last--1 or 2 hours?--all day? Then determine the absolute basics of what you wish to get across if, for example, the attendees are mostly those who gig part-time, such as warming up, relaxation is good to begin with, and then go on to very basic vocalise. Then perhaps correct breathing, and exercises to help them understand correct breathing. What is good phrasing? How does one emphasise the important word of a phrase. Basics of expression. You could have a whole day in just these subjects. i.e.,
*I* love you. i *LOVE* you. i love *YOU*.
I was recently asked to give a 2 hour workshop on singing harmony. JUST singing harmony. Most are not pros, and most need the basics, keep that in mind.
    As for warm-ups with a group. . .many years ago when I was still in music school, I overheard a member of the choir I had directed for 2 years comment to a new member, ". . .you can tell that she is really building a program here! Some of us complained in the beginning about the warm-ups, but we have all noticed that our voices are much stronger and we have much better breath control since she has been our choir director." Think about it.