I'd avoid OAuth.
Informal, per-server accounts seem fine. Why not start with a pragmatic solution - like a password to register with the server. If you know the passphrase you can join the server. The server can pass a private session key to the client — if the password changes, regenerate the key.
Then allow joining the server with a unique username / password. If you choose to manage your identity within the client - the client could do the account configuration for you — but that implies that the client has the password in the clear to submit to the server. Without central management it's hard to ensure folks have created a unique username – worst case, you'll have Alex as a user name and you'll join your friends server and you'll become Alex #2 when participating.