How would you ensure that everyone was involved in the planning and decision making process?

Before you start make sure that you are very clear on what you need to achieve. Hold a brainstorming meeting first to share the problem and then collect all the ideas, constraints and issues (risks and opportunities) to come up with a plan, then when you and the relevant stakeholders are happy with the result (good idea to have a preferred option and then some other options), send it out for review and comment (so you can collect changes from people who can't attend the next meeting) and then after week call a final decision making meeting that nobody leaves until a decision is made that everyone buys into - they may not agree with it, but makes sure everyone understands that this is the most appropriate course of action that everyone/the majority agree on. Make sure you have minutes for that final meeting - you may need them in future.