[Manas Mitra]

To join a forum, an user has to request to join the forum & Admin has to approve the request. Only after that the new user can participate in the forum. Is there any way that Admin can add an user to a particular forum?

Thanks in advance for any related information.

[Robkk]

Use a groups plugin or some membership framework plugin.

For Groups

BuddyPress (There might be a plugin to autoassign users to groups during registration, tons of extensions for groups feature specifically)

Private Groups

Groups with bbPress extension plugin

For membership {you might come across an issue using these plugins, but Robin has a plugin that will fix that called WPfix1 and WPfix2, search the forums}

S2member

Paid memberships pro with bbPress extension

There are probably other plugins that I either do not know or haven’t listed that work with bbPress. So do a search and see what works best for your site.

[Manas Mitra]

Find a plugin BP Admin Actions (https://wordpress.org/plugins/bp-admin-actions/) to do this job.

[Robkk]

Thanks for sharing what you chose to use.

There are a number of great BuddyPress plugins that can help manage users to specific groups.