1. You would need to speak to WP and or Joomla - or have a search here.
1a. If you've seen 'plugins' that you think may do the job you want you should speak to the plugin author.
1b. That would need an extension unless the WP/Joomla plugins as in (1) above can handle that?
2. Not sure what you mean by 'class' but the permissions in phpBB3 will handle private forums etc.
2a. Once again, this would require an extension.
I suggest you have a search of the extensions forums
to see if there's anything that suits your purposes and, if you don't find anything you can make an extension request. The extensions writers discussion is also a good source of information. There's also a lot of documentation about such as the 3.0/3.1 User Guide
and if you have a specific question a search, either here or Google, very often finds something, there's not much that hasn't been asked before. There are no threads in phpBB they are topics.