Passing info to another site

Have a site and wish to link to another site and indicate with a token or something a special situation. The second site would allow access to certain pages based on the passed along info. I'm new at this so the simpler approach the better. An example would go something like this: A user logs on and is authenticated on the first site then may link to second site indicating user has been authenticated thus the second site would allow certain pages to be accessed.

Any thoughts appreciated.

Thanks
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