Sometimes they'll make a post on the site with instructions.
Sometimes they do not.

Sometimes this twitter account has info:
https://twitter.com/rootsjamsession?lang=en

But hasn't been updated in years.

Sometimes you gotta find out where it is then go down and try to get in with your wits, or your luck.

This year it looks like Citi bank card holders could have entered:
https://twitter.com/rootsjamsession?lang=en

Sometimes The Roots facebook account sends out details:
https://www.facebook.com/TheRoots/

I've been lucky and been able to get in 4 or 5 times? They've done 13.