This is entirely up to you, as you know your clients best. However, we've seen the most success by following this rule: Get the key decision makers in the room. Depending on the scope of the engagement, you could be having discussions that really need to include the highest level executives, who are most able to speak to the categories. If you can get the senior leadership team, that's ideal. You want the Plan session(s) to be full of strategic, high-level thinkers who are empowered to make decisions and know the direction of the company. Think C-suite. We suggest you encourage any out of town clients to travel in for the session as it's not ideal to do this over the phone.
For a smaller scope like a project, we suggest you invite the core team on the client side who is requesting the work. For example, Marketing lead, Technology lead and/or Product lead depending on your type of project. You’ll want those who are leading the project with the knowledge and expertise to best determine what will make the project successful.