As I understood, you have 2 servers - first is a community forums which manages user credentials (OAuth2), and second is a game server, and you wish to authorize users across these two servers with a NodeJS client (similar to how Twitter or Facebook does it), correct?
I am very familiar with OAuth protocol and an expert with NodeJS, and if I misunderstood anything, I am sure we can figure details in the chat.
Looking forward to it.
-Art.