I'm not sure we ever got around to a general guide, that I recall.
If you use the recent topics tab to browse the forum, you can see the various conversations that have been active recently. If you browse all topics you'll find years of older stuff, too, sorted to some extent by general topic (though we don't have a huge division of topics like some forums do).
Using the "Share" button on the very top of the page you can friend people and send private messages.
The events link at very top and/or the hangout tab were for organizing Google hangouts, which have happened on occasion - for a while they were kind of popular, but lately haven't happened. But you are free to come up with a theme and date and organize one!
On the right side is a reading list, which is a so-far very short list of articles various people have written about various things.