The planning meeting builds consensus and commitment among the attendees. Someone joining the team later can't have the same level of commitment. If your new player isn't expected for at least 90 days then you should weigh the value of starting execution of your plan today vs. the additional commitment of your new player. The new player quickly gains an understanding of the major issues, other team members thinking and motivation, the overall direction of the organization, and the leadership role they are expected to execute. They leave with specific objectives and 90 day action steps that help speed their integration into the organization.
Strategic planning is primarily a brainstorming session.
You don't often go into a planning meeting in the computer business and come out in the shoe business. Strategic planning is dealing with the twin issues of communications and focus. The process is not about coming up with entirely new, off the wall ideas but rather thoroughly listening to, understanding, synthesizing, and then prioritizing the current set of hopes, dreams, and ideas. Intense, facilitated discussion and prioritization of issues clarifies where the organization is today. Developing and articulating vision, mission, and strategy establish when we want to be in the future.