bbPress Notify (No-Spam)
bbPress Notify (No-Spam), aka bbPNNS is a powerful notification plugin for bbPress designed to send out new topic/reply alerts to all users belonging to selected roles as well as to subscribers of forums and topics.
Paid Memberships Pro
Paid Memberships Pro is one of the most popular Membership plugins with a huge ecosystem to connect it with other software. Among other things, it (or some of its add-ons) allow you to control which members have access to which posts, forums, or topics.
This extension plugin integrates bbPress Notify (No Spam) with your Paid Memberships Pro (both of which must be installed and active on your WordPress site).
Use this bridging plugin to reinforce the notification rules in bbPress along with the membership access rules in Paid Memberships Pro to ensure the right recipients get properly notified.