the person who is ultimately responsible that the work be completed. This may be the person who actually does the work or directs others to do the work.
the person who has the legitimate authority to approve the adequacy of the deliverable (work), budgets, resource allocations, assignments, purchases, and so forth
the people who can provide administrative services or coordinate the logistics that can play a supporting role in implementation of the work.
the people with technical expertise and / or capability necessary to complete the work by supporting the project. This requires two-way communication.
the people who need to know the status of the work or the decisions that were made, whether it be a matter of courtesy or to help them better schedule their own work or the work of others. This requires one-way communication.