A mission statement is a requirement for any kind of official organization (such as a non-profit corporation), but it's very useful for informal groups as a more structured version of the elevator pitch?
A mission statement should include:
Why?
The purpose or essence of the organization; the change that will occur because of its existence.
What?
A brief description of the services provided by the organization that lead to the change or impact.
Whom?
The target population served by the organization.
Who?
The staff and/or volunteers that deliver the service-outlining any distinctive characteristics.
Where?
The geographic parameters that define your service area.
How?
The method by which the services are delivered.