Classical MAC coding schemes assume independent transmitted signals, because the given messages are assumed to be independent and the remote transmitters cannot cooperate. However, when the messages are correlated, or in the extreme case of fully cooperative transmitters, a better performance may be achieved. We explore the performance gain of this collaboration and its connection to transmission synchronization. For totally asynchronous transmitters we prove that transmitters collaboration has no performance gain. This result is proven for discrete-value channels. For continuous-value channels we focus on the collaboration gain of the AWGN MAC, indicating that it is bounded by a single bit and a factor of two. We suggest that this collaboration gain diminishes without synchronization as well.