Who should be involved depends ... on the thought process
Folks who focus on positioning often use academic language.
Lets start with an assumption about your company's leader's style: positioners or adapters. Are they rational, who study & model, prioritize, act, review? Are they adapters who are experimental, who trying many variations, go with what sticks, or intuitive who see an opportunity, grab it, move fast, never look back? A buzz word for adapt is "emergent." Careful, these aren't mutually exclusive. The tension between positioners and adapters is the need to innovate. Positioners want incremental improvements to high margin products and adapters want to adapt to market changes quickly, both want to grow the business.
Rational - involve as many folks as you can ...
Most strategic planning team use a rational approach to work through market and internal assessments, align around a vision and make course corrections to position the firm. To overcome the worship of past successes, fears of "sunk cost", seeing only what one wants to see, a rational approach is critical to get folks to prioritize, decide and commit. Folks have to acknowledge that they are responsible for the results. If you prefer a rational, very structured approach, then you want many people involved, both the people who you consider your key players and a good representation of the adapters, those closest to the customer.