A communications matrix can be a tool to facilitate your communications plan or it can be your entire communications plan. The matrix is a simple concept that lists the resource names on one side of a spreadsheet and the communication documents on the other. Then, you mark which resource agreed to which communication. Here is an example:

COMMUNICATIONS MATRIX

Placing this into your overall project plan can clearly state your communication intentions on the project. It can also cut down on unnecessary communications to team members.