Normally you would achieve this by creating a user group
sudo addgroup writewww
add the user(s) who should have the access level to that group
sudo adduser username writewww
and then set the access premissions on the folder
sudo chown -R :writewww www
sudo chmod -R g=rwx www
(This will change the group owner of the www folder to
writewww and then give the group read, write, traverse access on that folder)
If you want more control, you can user access control lists, but they're much more complicated.
Edit: That said, many Linux distributions' apache package now creates an apache group by default, so you can simply add users to that group. It would be great to see that in the Unix world.