How do I list the members of a group?

Use the commands:

getent group groupname

or

getent group groupname | awk -F: '{print $4}'

You can do

members YOUR_GROUP_NAME

and it will list all the users in the group YOUR_GROUP_NAME.

If it's not installed by default:

sudo apt-get install members

One more way to check all the members of a group is by checking the /etc/group file which lists all the groups and its members

Example:

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:syslog,nikhil
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:
fax:x:21:
voice:x:22:
cdrom:x:24:nikhil
floppy:x:25:
tape:x:26:
sudo:x:27:nikhil
audio:x:29:pulse

The first string (separated by :) specifies the group name and the last string specifies the user added to this group.