Invite only/closed groups

This plugin if Self hosted might help with some of the functionality your looking for.

Combined with groups and category security settings you can have topics in a category that are managed by the Op. Use groups to limit who can can create topics for discussion can le

  • Audi Australia admin Create/reply/see
  • Audi Australia members See/Reply

Members in the admin group could also be owners in the General group

In Category settings there is a setting to show a message if a person cannot post. So if Category has security setting
Everyone See

You can have a pop up message that has a link to apply to a group.