The things you write that are inspired by the latest PMIN are supposed to be posted as journal posts, not comments. Comments are reserved for reactions to a given journal entry by a member. ( I have some newbie-ish friends that were unclear about this.)
And to actually "join" this community so that you can post, go to the profile page and right at the top of the page it says "to join this community, click here" and ya click there.
This then becomes another journal you can post to, just like you post to your own journal.