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Sometimes, picking the right people for the workshop is harder than running it. An external facilitator isn't the best person to decide who should be invited and who shouldn't; job titles don't say much about potential contributions.
But picking the right people can become as annoying as choosing tables for friends and relatives at your wedding: "Why wasn't I invited?" and "Why was I invited?" are two equally nasty problems to handle. There is no perfect way to make the selection work.
Maybe the best solution is to avoid taking on the responsibility of inviting at all and let human beings be responsible for their own choices.
Make people aware that you're running the workshop, let the information spread, and self-organisation do some magic for you.
Keep a pattern of open invitations: you need to keep the headcount under control for booking an appropriate room (but be ready for some last-minute joins).
Secure the key people you really need, and let their presence be the main driver. For other participants, let commitment be the main driver; you may discover unexpected sponsors within your organization. In some cases, you may need to keep some aces up your sleeve: depending on the company culture, some people may be afraid to be in the same room with their bosses.
Selling the workshop format is not your goal. Don't use the invitation to explain what EventStorming is. Your workshop has a purpose ; EventStorming is just one of the possible tools to achieve it.
You may eventually need a few people who won't consider the event worth their time (see also Dungeon Master ). In this case, you may want to ask for their participation explicitly as a request for help. If you fail, you will have gained valuable insight into where your key problem might lie.
Expect quite a few questions about the event; keeping some of the answers public in a FAQ can help keep your answers consistent.
Please remind that 'open' doesn't mean half-hearted: the ones joining the workshop are supposed to give full attention. Getting in and out, and asking for summaries and recaps, negatively impact everybody's experience.
Size Matters
The organisation's size clearly influences the strategy: Open Invitation plays well in mid-sized organisations where the number of people involved is still relatively manageable. Twenty to thirty people is a common number for a Big Picture workshop, and it maps relatively well with a single line of business, but every organization is different.
Sometimes, there is no way to make the first workshop perfect. Maybe the only goal of the first workshop is to make the right one possible. See also Iterative invitations and Leave stuff around .
Manage selection biases
Volunteering for the workshop may provide you with an unbalanced audience. It's probably still the best possible outcome, but it could affect specific stages, such as Arrow Voting . Make sure that you don't give too much political significance to the collective opinion of only one side of the organization.