cut -d: -f1 /etc/group | sort
cat /etc/group
cat /etc/groups | grep <groupname>
# Example:
cat /etc/groups | grep sudo
# Output
sudo:x:27:<user1>, <user2>, <user3>...
# sudo : group name
# x : password (encrypted for security reasons)
# 27 : could be another number, represents group ID
# list of users in group
groups user
groups
groups [user name]
# [user name] : (optionnal) the user to list the group of
# default is the one executing the command.
cut -d: -f1 /etc/group | sort
groups $username
groups userName-Here