Just curious as to how other people go about it.

Well, 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.

Thanks, I appreciate it!

I was mostly wondering how you write with someone else on a story, but thanks for the details of this site, too.

Put a request out in a society with a gist of what you want to write and see if anyone wants to collaborate 😊

Alright, then. Thanks.