adding a secondary group for user via CLI in Lion is not working
Superfical answer but perhaps it may help:

groups username 


tells me that this command is deprecated, and I should be using

id -Gn 


or 

id -p


I then get the list of groups that i belong to in its entirety. 

man id 

dsmemberutil seems more like a way to query the data store, which I was actually looking for,so I hope this helps you. Thanks.