Millimeter-wave-based extended target tracking has attracted extensive interest recently because of its privacy, high precision, and low cost. This paper concentrated on crowded indoor situations and presents a novel method for group tracking. First, the proposed alpha-extended Kalman filter and the group association were carried out, which can constantly estimate the target expansion and the number of reflection points, consequently modifying the measurement noise and covariance estimation. Then, to initialize the actual targets, we employed a density-based spatial clustering approach that includes false target suppression. After the targets have been updated, the track re-association and estimation procedure is conducted to account for the unanticipated break of moving and near-static targets. Finally, various experiments involving fewer than 11 participants were designed to assess the robustness of the method. As a result, continuous and steady tracking results, as well as high counting accuracy were obtained.