It is best to educate them from your perspective on all aspects of the game. It is just as important to consider their input on the product. Marketing, for example, can help in defining the main character for the target market. They should also be consulted to determine when demos and marketing materials will be needed from the developer. Marketing can also assist in providing market research and gathering data through focus testing.
Sales can provide valuable feedback from retailers.
The more educated and familiar these parties are about the product, and the more you consider their input, the better the chance for the success of the product. Undoubtedly you will need to let go of some of those great ideas you had at the beginning of the project. Ideally, the milestones have been prioritized such that the ideas to be left on the shelf are not core elements of the game. At the Alpha stage of development, it is time to reassess the remaining work to be done, give consideration to new ideas that have germinated over the course of the project, and incorporate them into the schedule if they are viable. This may help to motivate the team through to the end.