Funny you go this way, I'd personnaly use the dseditgroup command with
something like this :
dseditgroup -o edit -a lanadmins -t group -n /NetInfo/DefaultLocalNode
admin
Well there are a couple of reasons I do it that way.
Firstly I have an existing NetInfo database that gets pushed out via
radmind, and it's not upgraded from "legacy", so dseditgroup won't
work, even though nesting the group manually does work.
Secondly due to NetInfo being a pain in the arse to manage
atomically, I have a whole raft of NetInfo properties that I set at
boot on my images, and they differ for each machine, so I set them
all in a consistent manner.
I should have suggested this though. Ideally Workgroup Manager should
let you add groups to the local NetInfo groups when you're viewing
the NetInfo store as well.