Singel Sign-On to my WordPress site

Im looking for a solution where a client want to log in it's user automatically to my WordPress site. The solution seems to be Single Sign-on.

I have seen this thread Create API for single sign-on with 3rd party site but it seems to be the other way arround, if Im not wrong? That the user want to be able to login to to a 3-party site with users on his WordPress site.

Am I on the right track and is there maybe a solution out there? Plugin?

The standard we should use haven't been set yet, so if there is multiple ways to do this that isn't a problem.