How can I add an existing user to an existing group via the command line on a Mac OS X machine?

dseditgroup -o edit -u your_admin_username -p -a the_username_of_user_you_want_to_add_to_group -t user the_name_of_the_group_you_want_to_add_user_to

If I wanted to add myself to the "www" group I would simply run the following command:

dseditgroup -o edit -u johnerck -p -a johnerck -t user www

Run the groups command to see that you've been added:

groups