No, it isn't incredibly simple or very obvious lol, so it's not you. :)
If you want to join a forum, you'll have to find a member of that forum, click/tap on their name, and then go to their profile page (the click/tap will take you there). On their profile page, you should see a widget/module titled "Communities". Next to the community name, you should see "Join". Click/tap on that, and you can join the community.
If you just want to watch a community, and follow what is being posted, you can scroll to the bottom of the topics list (all the posts that others have posted), and click/tap on "Watch this community", and updates will appear on your Watch List.
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