From http://serverfault.com/questions/124800/how-to-setup-linux-permissions-for-the-www-folder

setup linux permission for www

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After more research it seems like another (possibly better way) to answer this would be to setup the www folder like so.

sudo usermod -a -G developer user1 (add each user to developer group)
sudo chgrp -R developer /var/www/site.com/ so that developers can work in there
sudo chmod -R 2774 /var/www/site.com/ so that only developers can create/edit files (other/world can read)
sudo chgrp -R www-data /var/www/site.com/uploads so that www-data (apache/nginx) can create uploads.
Since git runs as whatever user is calling it, then as long as the user is in the "developer" group they should be able to create folders, edit PHP files, and manage the git repository.

Note: In step (3): '2' in 2774 means to 'set Group ID' for the directory. This causes new files and sub directories created within it to inherit the group ID of the parent directory (instead of the primary group of the user) Reference: http://en.wikipedia.org/wiki/Setuid#setuid_and_setgid_on_directories