As you develop and build coalitions with the decision team, extended team, internal departments and suppliers, and external suppliers, you, in essence, are establishing the basis of an "information network." The information network will become an excellent tool for gathering, sharing, and disseminating information instrumental in developing the ISD model and strategy. Once you establish a relationship, the only thing between how closely you want them tied to the project and how closely they feel tied to the project is the quantity and types of information being shared with them, as well as the frequency of communication.
At the risk of sounding redundant, overcommunicate. Keep the flow of information open and turn any feedback around as quickly as practical. This accomplishes a few things. It reinforces that any and all feedback is considered and valued, therefore making those that have contributed feel even more closely tied to the output from the project team. It also keeps everyone "up to speed" on the latest information, reducing the need to provide "level-up" sessions. You will reap many benefits through continuous evolution and refinement of a strong information network.