Well, for the mechanics of this site, you can go to Penana Home and click on Collaborate under the search bar. Under there, there should be checkboxes for "Betareader," or someone who edits drafts before they are published, "Co-writer," which is a coauthor, or "Idea Contributor," which is someone who can comment on drafts before they are published.
Select whichever positions you are interested in being and it will show you all the stories that are looking for one of those positions. Chose a story you are interested in and then Join them!
I hoped this helped. If you're on mobile, it's different, and if you want me to tell you how to do it there, let me know.
If you're just wondering how to get people to notice you, this is a good place to visit, and other places to reach out would be the "Find Your Writing Buddies" society, or horror and humor societies.