Finding all the members of a group is an occasional requirement and while there are a number of ways to do this by parsing the /etc/group and /etc/password files, Debian/Ubuntu come with a simpler solution that performs all this skulduggery for you. This is the members function that can simply be installed using sudo apt-get install members . Once this is done, members of a group named foo can be listed using:members foo.
The function also supports a few options to differentiate between primary and secondary group members. The man page is as follows:
MEMBERS(1)
NAME
members - outputs members of a group
SYNOPSIS
members groupname
DESCRIPTION
This manual page documents briefly the members commands. This manual page was written for the Debian GNU/Linux distribution.
members is a program that sends a space-separated list of secondary member names to its standard output.
OPTIONS
The programs follow the usual GNU command line syntax, with long options starting with two dashes (`-'). A summary of options is included below.
-a, --all
Show all group members on one line. This is the default.
-p, --primary
Show only primary group members.
-s, --secondary
Show only secondary group members.
-t, --two-lines
Send two lines to standard output. First line is primary members, second line is secondary members. NOTE: This always displays two lines, even if there are no members at all.
-h, --help
Show summary of options.
I'm not sure if this package is available for other distributions. It probably is.
Hope this helps.