To follow a community, copy the ID of the Fediverse community, and on nodeBB click the magnifying glass on the right side to search and paste in the community ID in there (and delete the exclamation mark, like television@piefed.social)
Open the community, click "Not Watching" to choose either "Watching" or "Tracking" and then new posts/comments will be synchronized in, and you should be able to join in on the discussions!