  In this paper, we investigate how to detect intruders with low latency for
Active Authentication (AA) systems with multiple-users. We extend the Quickest
Change Detection (QCD) framework to the multiple-user case and formulate the
Multiple-user Quickest Intruder Detection (MQID) algorithm. Furthermore, we
extend the algorithm to the data-efficient scenario where intruder detection is
carried out with fewer observation samples. We evaluate the effectiveness of
the proposed method on two publicly available AA datasets on the face modality.
