How To Create Groups in OS X?

A question for all your OS X/UNIX geeks:

I had to create a new group on Kelly’s Mac so that she could write to an NFS share. After poking around NetInfo, I created a group I thought would work but the group didn’t show up on Kelly’s group list. Finally, I duplicated an existing group and changed the values accordingly. That seemed to work.

How does one add users to a group using NetInfo? I can’t figure out how to get the user list to show multiple entries.