tracking USER_ID

[UsmanJ]

I want to be able to push the user id to ga (so ga can distinguish between new and old visitors).

Normally in JS it would be along the lines of

 ga('create', 'UA-XXXXXX-Y', {'userId': USER_ID});
  ga('send', 'pageview');

How can I modify the constructor to be able to do this?

[abmultimedia]

Also looking to do custom variables server side. Any insight would be much appreciated