I spent many hours recently looking into this again. The problem with the post or comments box not showing up is only for events which are set for groups. The events which are not attached to groups can be commented on by those going to the event. I have checked every single permissions on pages, blocks, and navigation. You have to be the site administrator to make the initial post in events which are designated for specific groups. I still have not found a solution. When I enable the info for who can post, I get an error (see attached image).

I can get as far as adding the post and comments to the group event page. But they just stall and are not loading. See the attached screenshot.

I have found a workaround solution until the Events module integration with Groups can be fixed. Members who are creating an event for a group (who are not the site admin) can use the "Share to context" button on the timeline post for the event (You can do this with a new post. But if you delete the original new post, the shared post will disappear from the events page.) After clicking the button, select to share it to the new event. This will put a copied post with the event banner inside the event page so that all of the attendees can comment and talk. It's not ideal, but it works.