Manage Mac user and group accounts from the command line with dscl.
# list all users
dscl . list /users
# list all groups
dscl . list /groups
# list information about each user
dscl . readall /users
# list information about each group
dscl . readall /groups
# add user `root` to group `kmem`
sudo dseditgroup -o edit -a root -t user kmem
# verify user `root` is in group `kmem`
sudo dseditgroup -o checkmember -m root kmem
# remove user `root` from group `kmem`
sudo dseditgroup -o edit -d root -t user kmem