I am running an nginx server and a node express web server, using daemontools, setup to communicate over Unix Domain Sockets. There's just a few problems:
Maybe I am too late.
As a complement of your own answer there is a solution not having to add the nginx user to the node group.
Create a directory only for the socket file, assign it to the node user and www-data (or whatever group the nginx is) group and set the group-id bit (SGID) on that directory.
mkdir -p /var/lib/yourapp/socket chown nodeuser:nginxgroup /var/lib/yourapp/socket chmod g+rxs /var/lib/yourapp/socket
All files created inside this directory will automatically be owned by the nginxgroup group.