Alternatives to authentication

I'm writing a management system that will, initially, only have 3 users but there's one more guy who needs access. The system integrates with our accounting package and they need to be authenticated against it so it makes sense to use their oAuth.

However, there's one more guy who will need access to one set of screens but we don't want to give him access to the accounting package and therefore he can't authenticate. He has literacy problems so it needs to be really simple.

My options...

Alteatives to authentication WordPress ...