We investigate the physical layer security of synchronous multiple access transmissions using the Alamouti space-time block code in fading channels where multiple users communicate with a single intended receiver in the presence of an eavesdropper. We propose an artificial-noise-aided technique to secure the transmissions by having the Alamouti users collaborate with each other, without exchanging information, to degrade the eavesdropper's channel. Unlike previous work, which assumes that the transmitters have complete knowledge of the legitimate as well as the eavesdropper's channels, our proposed technique requires no communications between the users, minimal knowledge of the legitimate channel, and no channel knowledge regarding the eavesdropper.