newgrp asks for a password

I don't understand why newgrp prompts me for a password when I want to change my group.  I don't even know what password it is asking for, but it is certainly not mine (user clime). $ id uid=500(clime) gid=501(www) groups=501(www),500(clime) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 $ newgrp clime Password: <--- huh?! My system is CentOS 6.3.