How can one define who has access to a private forum in PHPBB?

To define who has access to a private forum in PHPBB, you can use the built-in permissions system in PHPBB. You can create a new user group specifically for the private forum and assign permissions to that group to control who can access it. By adding users to this group, you can restrict access to only those users who are authorized to view the private forum.

// Define the user group for the private forum
$group_id = 5; // change this to the ID of the user group for the private forum

// Add users to the user group to grant access to the private forum
$user_id = 1; // change this to the ID of the user you want to grant access
$user->add_group($user_id, $group_id);