How to give access to my user and _www
To give a user or a group full access to a folder use

chmod +a "user_name allow list,add_file,search,delete,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,writesecurity,chown,file_inherit,directory_inherit" /path/to/folder


You may replace user_name by a group_name.

To remove the ACL simply replace the +a by -a.

It's similar for a single file - the attributes are different though:

chmod +a "user_name allow read,write,execute,delete,append,readattr,writeattr,readextattr,writeextattr,readsecurity,writesecurity,chown" /path/to/file


Insert a -R for all files in a folder:

chmod -R +a "user_name allow read,...