Want to add a user to a specific group using the command line? dseditgroup is your friend! Add users, or groups, to a group you create or system groups which control access to services.

Make sure to insert your local admin’s short name (localadmin) and the user (username) or group (groupname) you’re trying to add.

Remote Login (SSH)
User: dseditgroup -o edit -n /Local/Default -u localadmin -p -a username -t user com.apple.access_ssh
Group: dseditgroup -o edit -n /Local/Default -u localadmin -p -a groupname -t group com.apple.access_ssh

Screen Sharing
User: dseditgroup -o edit -n /Local/Default -u localadmin -p -a username -t user com.apple.access_screensharing
Group: dseditgroup -o edit -n /Local/Default -u localadmin -p -a groupname -t group com.apple.access_screensharing

Print Administrators
User: dseditgroup -o edit -n /Local/Default -u localadmin -p -a username -t user _lpadmin
Group: dseditgroup -o edit -n /Local/Default -u localadmin -p -a groupname -t …

Source: Mac OSX Hints