How to allow a group to access a folder? I want to allow the _www group to write to a folder.

I clicked on the folder to go to the "Get Info".

At the bottom there is a "Sharing &amp; Permissions" section with a "+" to add users. However it does not show any of the system groups such as _www.

Is there a way to enable showing hidden users and group in the GUI?
Or do I have to do it via command-line instead?
It seems the GUI allows to set permissions for several users and group, which I like, while the command I believe only allow 1 owner and 1 group?
Apple hides anything it considers a "system" user or group from the GUI.  There's probably some way to disable that, but I generally use the command line for system-related stuff.

The command line version comes in two flavors:  there is a basic permissions structure with a single owner, a singe group, and everyone else, then there are POSIX ACLs.   Interestingly, Apple took the route of modifying chmod to support ACLs instead of shipping the standard getfacl and setfacl tools.

$ sudo chmod +a 'group:_www allow add_file,add_subdirectory,list,search,delete_child' /webroot


You may also want the file_inherit and directory_inherit permissions to apply the ACL to created files/directories.  See chmod(1) for more details.

The -a option to chmod removes ACL entries.