A year ago, before I started working for Discourse, I did quite a lot of work on the plugin’s discourse_topics shortcode. I’m quite happy with the way it works. As far as I know, that is the shorcode that people are using. I’m unsure about the current state of the discourse_groups and discourse_prefilled_message shortcodes. I’ll take a look at them as soon as possible.
The topic that you linked to is very out of date. I’ll clean it up this week.