There is a rake task. It’s a little clunky (I’m not sure what about user passwords, for example?) but I think it works.
That gets the categories, which are the collections of topics that you want. I think it also pulls over the users who have created posts. I’m not sure about other users that have not created posts.
I don’t really recommend it, but if one group got really big and wanted to spilt off you might do it this way, or you might just restore the whole database and delete the categories you don’t want. It’s hard to tell which would be easier without seeing the actual data, which do not exist.