Nitrogen has one lone pair which can serve as a hydrogen bond acceptor. Note that the mode of hydrogen bonding in ammonia dimer differs greatly from the one seen in water dimer! In water dimer there is one hydrogen bond and it is rather "linear", that is, the acceptor atom and the H-O bond of the the H-bond donor are nearly collinear. Not so for ammonia. Here two bend hydrogen bonds are formed. In bend hydrogen bonds the acceptor and the H-X bond are not collinear. While each bend interaction is not as good as a linear interaction, two bend interactions are better than one linear interaction in the case of ammonia. (Why water and ammonia differ in their dimer structures is a more difficult matter.)