Syncronising all network users on a subsite

I want to create a forum using bbpress (or whatever plugin) on a subsite. I would like all the users of the multisite to be, automatically), members of the forum site as well. I've created the main domain as a master site, but is not working as I wanted, because they're all users of the network, not of the main website.
Is there any way I could synchronize network users with a subsite?
I tried the solution found here:
but every time I Network Activate the Multisite User Management plugin I get a blank wp-admin/network/settings.php - looks like a conflict with something, so I can't use this plugin.
Thank you for all your support.