How to allow a group to access a folder?
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.