First, to find which groups you are currently in, you can use: % groups which promptly listed all the groups, except the one I needed. How do you add the user?
That's where the dseditgroup command comes in. It's a command-line tool which can perform certain editing operations on groups. It has an explanatory man page, and I used this command to add a username to a group. The parameter -u yourusername is your admin account, and the -p will prompt for the password.
% dseditgroup -o edit -u yourusername -p -a username -t user group.
[crarko adds: I tested this, and it works as described. You can consult the man page for dseditgroup for more information.]