  The General Gaussian Multiple Access Wire-Tap Channel (GGMAC-WT) and the
Gaussian Two-Way Wire-Tap Channel (GTW-WT) are considered. In the GGMAC-WT,
multiple users communicate with an intended receiver in the presence of an
eavesdropper who receives their signals through another GMAC. In the GTW-WT,
two users communicate with each other over a common Gaussian channel, with an
eavesdropper listening through a GMAC. A secrecy measure that is suitable for
this multi-terminal environment is defined, and achievable secrecy rate regions
are found for both channels. For both cases, the power allocations maximizing
the achievable secrecy sum-rate are determined. It is seen that the optimum
policy may prevent some terminals from transmission in order to preserve the
secrecy of the system. Inspired by this construct, a new scheme,
\ital{cooperative jamming}, is proposed, where users who are prevented from
transmitting according to the secrecy sum-rate maximizing power allocation
policy "jam" the eavesdropper, thereby helping the remaining users. This scheme
is shown to increase the achievable secrecy sum-rate. Overall, our results show
that in multiple-access scenarios, users can help each other to collectively
achieve positive secrecy rates. In other words, cooperation among users can be
invaluable for achieving secrecy for the system.
