[PrincessL]

I already have a website up and running for 5 years.
Now we want a Forum. It will be private by invite only.
Can I add the plugin to my website?
Or do I need to create a brand new/unique WordPress install in a different folder and install the plugin?

[Robin W]

if only your invited users have login, then just set the forums as ‘private’.

If lots of users have login and you want only some to see the forums, then use bbpress and this additional plugin

Private groups