Physical layer security is a promising approach to enable secure communications by considering the characteristics of wireless channels. In this work, we consider a multiple antenna multiuser system with an eavesdropper which is passive and its channel state information (CSI) is not available at transmitter. We we add artificial noise (AN) in the null space of legitimate user to disturb reception of eavesdropper. For the sake of ensuring secure communication, CSI has great impact on system performance. However, in order to estimate the channel of all users at transmitter, sending pilot signals from all these users increases the system overhead. We perform a threshold based user selection at the receiver side to reduce this overhead. Also, we investigate the effects of the proposed user selection on the secrecy capacity.