How to implement single sign-on

Hello there!

I have this new application I’m writing for a client.
Currently, there is an older application where they already have a login and use it regularly.

This new application will have to use the same role system that the old application did. This means that if, in the old application, they are admins, they will have access to admin content in the new application.

The way I thought of solving this is by using Guardian, but it still feels kind of clumsy to me. What is the best way to attack this?