Looks like it is not even documented on the samba website itself, or did I miss it?
Seems pretty straight forward...
usershare_acl=-The user you want to give ACL's to-:-ACL Modifier-
Seem's fairly well documented to be put up as a question that you are answering for yourself the same minute you ask it.... :\
Best hit I got is from: http://devarthur.blogspot.de/2008/05/integrating-ubuntu-hardy-heron-804-with.html
To get the SID of the a group use:
SID "S-1-1-0" is Everyone
The access modifiers after the group SID are as follows: