Using Single Sign on with WordPress

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I use both WordPress and C5 on many sites. This allows me to use the best of both worlds (in my opinion). I am now working on a site that will have a registration and would like to "share" the users across the two. Has anyone done this?

I imagine it involved adding the user/ password/ info into both databases so if you log in on one, the other is in sync.

I also see there are WordPress to PHPbb plugins for Wordpress.. just hoping there is one from C5 to WP.

Thanks for any suggestions/ comments (or solutions)