Have a look at the manpage
http://www.hmug.org/man/5/group.php
Like in "passwd" file, the 'x' field is the password field. It says 'x' because the file needs to be readable by non-root users too, and giving out (encrypted or not) passwords is a security risk, so usually there are no passwords in the field. The case is somewhat same with 'passwd'; the passwords are nowadays saved in 'shadow' file instead of 'passwd', and 'passwd' merely states 'x' in the password field to tell that the actual password is in 'shadow'.
