I was using a script to try and add users to a local group today in an automated build script. I was sent a script that worked, but when I plugged in my values, it didn't. I always got a script syntax error, like one of my parameters was not in the right place, or had spaces or something. Well the group name did have spaces, but that wasn't my issue.
It turns out that there is a documented Windows bug (http://support.microsoft.com/kb/324639) that limits the username to be added to only 20 characters. Any more and it won't run.