For the workshop, you will always need to have two types of people: people with questions and people with answers.

People with questions are developers and architects. Surprisingly, developers rarely attend any sessions that directly involve potential users of the software they plan to develop. We already touched upon this topic when we discussed the separation between problem space and solution space in Chapter 1, Why Domain-Driven Design. So again, having developers on such workshop is essential. Another group of people that some might expect to be those with answers, but in reality, they have more questions - these are business analysis or requirements analysis, whatever are they being called in your organization. Admittedly, they spend quite a lot of time together with potential users and customers, but usually, during the EventStorming workshop, they get new insights since this exercise is not one-to-one but for a group.

This group needs to study the information about the domain that is already available (general understanding, may be requirements or specifications if those were already made) and prepare questions.

People with answers are usually those who we call domain experts. But remember that they do not know all the details, and they also can have an illusion of knowledge. It is why you need as many of them as you can get to the workshop since these people are usually not easy to gather together in one room because of their busy schedule. It indirectly suggests that those people are generally quite high in the organizational hierarchy, but not necessarily you need to aim at getting the most person in each department. You might search for someone, who knows how things are done instead of those who only know how things should be done, based on some fictitious descriptions and standards.

This group could prepare by formulating problems better. Thinking what issues need to be solved and being more specific about these issues, might help during the workshop.