In a Wireless Local Area Network (WLAN), clients typically associate with the AP that offers the maximal signal strength. Later works on client-AP association then further take load balancing and fairness into consideration. Those schemes however are not directly applicable in a multiuser MIMO (MU-MIMO) WLAN since different combinations of clients result in different throughput of each individual client. Therefore, in this paper, we present a client-AP association algorithm customized for MU-MIMO WLANs. The proposed algorithm jointly solves the problems of client-AP association and MU-MIMO client grouping with consideration of channel correlation among clients. It hence allows a good group of clients, i.e., those with low channel correlation, to together associate with the same proper AP and achieve a high sum-rate. The simulation results show that our MU-MIMO AP association algorithm improves the aggregate throughput by about 11%-28% and 26%-45%, as compared to two common association approaches, i.e., RSSI-based and load-based schemes, respectively.