How to add user to a group from Mac OS X command line

Use the following commands to add user to a group:

sudo dseditgroup -o edit -a usernametoadd -t user admin
sudo dseditgroup -o edit -a usernametoadd -t user wheel

It’s also possible to do this with dscl, but to do it properly you need to both add the user’s short name to the group’s GroupMembership list, and add the user’s GeneratedUID to the group’s GroupMembers list. dseditgroup takes care of both in a single operation.