Forum topics can be created. Due to the theme not being one typically used with the BuddyPress plugins, there are a few quirks.

First off, to be able to post threads, the group’s email options must be set to something other than ‘No email’. Admins can change this in the Group Settings. I know for a fact that Weekly Updates is a working option.

No doubt, you have tried to post a thread and have gotten some official looking ‘Oh no you didn’t?!!??’ error message about sent headers and such. Disregard that, hit your browser’s back button, click on the forum tab and you will see your thread.!

Responding is much the same, except you are seeing a broken link message. Just hit the back button and you are good