Harris has hit on the same points I consider. Another point is my goal. I always have a goal I want participants to achieve for each workout. For example, to build strength, to improve flexibility and ROM, to work in 75-85% HR max, etc. Each session has a goal and depending on that goal can also depend on how I structure my sessions and rest time.
Have fun with it. Be flexible and able to change the intervals quickly if you see its not working for the majority of the group. Also, consider having the group broken into teams if you have people of different abilities. When working with a group, I find its best to come with a plan, and to remain flexible enough to change that plan if you find it isn’t working.