WPMU DEV FORUMS Plugin
Give each site on your Multisite network a simple tool for creating its very own forums that can be embedded directly into any page or post
WPMU DEV FORUMS Plugin lets you add message boards to posts or pages across an entire Multisite network.
Add a message board to your site in just a few clicks and encourage community participation.
Include a title, brief description, set basic styling options and embed the automatically generated code into any page or post.
Please visit official plugin page for more information