I was at a vendor site yesterday, discussing how they were going to satisfy our needs on four upcoming projects.  There were four people in the meeting:  the technical lead from the vendor, the product manager from our organization, a director from our organization, and myself.  Since I am the project manager, I had to take into account each perspective of each participant.  Communications grows exponentially, every time you add another person to a meeting or project.  A project manager needs to realize the complexity and manage it accordingly. The situation reminded me of a few questions that were on the PMP exam.  Communication Channels  can be calculated by using the following formula:  [N (N-1)]/2 where N equals the number of people involved.  Do not just memorize the formula, UNDERSTAND it.  Below is an illustration of the formula that should help you visualize it.

If on the PMP exam, I had been asked how many channels of communications existed in the meeting, I could either draw a picture with lines between the people or I could just use the formula.  Take my word for it, just memorize the formula and understand when it applies.