A wiki is a must.
It helps as an asynchronous communication media between ans inside the teams. People can share their tips (eg how do I compile this, how to activate traces ... ). It can be used to gather design decisions or changes...
People can ask questions to the whole team without clobbering other mailboxes.
It can also be used to grow the documentation.
There is a gazillion of wiki's, pick one depending on what you plan to do with it.
