Using the Alamouti space-time block code in conjunction with artificial noise (AN) is an effective approach to secure multiple access (MAC) transmission in the presence of eavesdroppers. Existing studies on this technique focus on the two-user transmission systems and single-antenna eavesdroppers. This letter proposes a novel approach that generalizes this technique to multiuser transmission schemes and multi-antenna eavesdroppers. Security gains are analyzed, and the minimum number of MAC users that should participate in generating the AN in order to secure their transmissions is characterized analytically.