  We consider the General Gaussian Multiple Access Wire-Tap Channel (GGMAC-WT).
In this scenario, multiple users communicate with an intended receiver in the
presence of an intelligent and informed eavesdropper who is as capable as the
intended receiver, but has different channel parameters. We aim to provide
perfect secrecy for the transmitters in this multi-access environment. Using
Gaussian codebooks, an achievable secrecy region is determined and the power
allocation that maximizes the achievable sum-rate is found. Numerical results
showing the new rate region are presented. It is shown that the multiple-access
nature of the channel may be utilized to allow users with zero single-user
secrecy capacity to be able to transmit in perfect secrecy. In addition, a new
collaborative scheme is shown that may increase the achievable sum-rate. In
this scheme, a user who would not transmit to maximize the sum rate can help
another user who (i) has positive secrecy capacity to increase its rate, or
(ii) has zero secrecy capacity to achieve a positive secrecy capacity.
