It isn't obvious at first how to list all the member of a group from a linux command line (I'm using Ubuntu Server 12.04 LTS)
Listing a user's Groups
It is very easy to list the groups that a user belongs to. Simply use the groups command.
This command will output the user name and a space-delimited list of all the groups that user belongs to, as follows.
user : user dialout cdrom floppy audio dip vido plugdev users fuse lp admin sambashare
Listing a Group's members (Users)
If you want to do the opposite, and list the members of a group, you can use the members command. The catch is: the command is not installed by default. Fortunately, it is simple to install:
sudo apt-get install members
Once the package is installed, you can issue the members command similarly to how you issue the groups command.
This will output a space-delimited list of all the members of the group, as follows:
member1 member2 member3
Unlike the groups command, it does not output the name of the group. You may use the optional arguments --primary and --secondary to list only the primary or secondary users within the grooup.
alternatively if you want a quick look here are the commands.
cat /etc/passwd cat /etc/group