Share Users across multiple WordPress sites and phpBB

I searched for this topic but couldn’t find anything recent that works on the versions that I’m using. My starting point is a single WordPress site using phpBB Bridge for phpBB as me forums. We’ll call Mastersite.com

For that installations I have this configuration and these users are shared.
Site: mastersite.com
Forums: mastersite.com/forums

What I want to do is allow the users of mastersite to be able to log in at subsite.com — subsite has it’s own content and it’s own stories, but the forums are central to mastersite. All additional WordPress sites will link back to mastersite.com/forums for discussion, but I still want the subsite wordpress blogs to allow users to log in with their existing IDs and comment. Over time, additional subsites will be added in this manner.

I’ve seen config.php hacks that are supposed to share another site’s user tables but I haven’t gotten this to work between my two wordpress sites, much less when I throw phpBB into the mix. I found a user sync plugin that copies the users but the passwords don’t work on subsite. (Shared Users Reloaded). It looked like it might have worked on previous WordPress versions but there hasn’t been an update in over 500 days.

Is what I’m asking for even possible? If so, can someone point me to a resource in how to set this up?

Thanks for your help.