  We consider the Gaussian Multiple Access Wire-Tap Channel (GMAC-WT). In this
scenario, multiple users communicate with an intended receiver in the presence
of an intelligent and informed wire-tapper who receives a degraded version of
the signal at the receiver. We define suitable security measures for this
multi-access environment. Using codebooks generated randomly according to a
Gaussian distribution, achievable secrecy rate regions are identified using
superposition coding and TDMA coding schemes. An upper bound for the secrecy
sum-rate is derived, and our coding schemes are shown to achieve the sum
capacity. Numerical results showing the new rate region are presented and
compared with the capacity region of the Gaussian Multiple-Access Channel
(GMAC) with no secrecy constraints, quantifying the price paid for secrecy.
