Set default group for user when they create new files?
If you want all new files in a particular directory to be owned by a particular group, just apply the setgid bit on it:
chgrp www-data /some/dir
chmod g+s /some/dir
If you have an existing tree of directories that you want to apply this behaviour to, you can do so with find:
find /some/dir -type d -exec chgrp www-data {} +
find /some/dir -type d -exec chmod g+s {} +
(if the directories are already owned by the relevant group, then you can omit the first command in both of these examples).