Write With Me is a platform that enables two authors to write a single piece of literature collaboratively. One author will take their turn to progress a document and pass it on to their co-author when they are done for the time being. So, only one person can add to the document at a time. You can also just write solo pieces here.
This form of turn-based writing allows each author to be at the helm of their work during their turn. The co-authors can benefit from discussing blueprints or details beforehand to ensure the finished document will be coherent, but turn-based writing can still introduce reasonable surprises since each author can bring new developments to a prose or poem that their co-author may not have considered. This also eases the burden on each author to carry the work forward on their own all the time and keeps things exciting for them, since probably half the details of the work are buried in their co-author's imagination.
Start a Post by just pouring the first few words of your idea into a fresh Turn 1 sheet, found in your Home or Profile page.

If its not quite ready yet, tap Create Draft to save the Post in DRAFTING status and pick it up later from the Posts tab in your Profile page. If you are ready to find co-authors, tapping Invite Authors will prompt you for the following details.

Enter a temporarily Title. You can always change it later.
Pick from a Rating of Light, Mixed, or Mixed & Strong (learn more about the expectations from each label).
Choose a Category (Fiction, Non-fiction etc.).
Adding a rough Synopsis can help probable co-authors improvise better.
Tags function as labels for genres, sub-genres, literary tropes, or story archetypes. Tags are vital for your co-authors and readers to find literature that fit their interests. Make up your own tag by typing and pressing the + button or pick from a list of pre-existing ones.
Lastly, your Post will by default fall under English literature, but you have to specify if you are writing in a different language.
When you hit Invite Authors again in this prompt, this shifts your Post to INVITING_AUTHORS status and temporarily publishes your Post to the feed (Collaborate tab in Home page) of other users, from where they can collaborate on your Post.

Prospective co-authors can now submit their own versions of Turn 2 (collab-submissions) for your Post. A prospective co-author will be able to see and edit only their own collab-submission.
From the Posts tab in your Profile page, you can go to the Dashboard page of your particular Post (by clicking on the Post Title). In the Dashboard page, you can see all collab-submissions made by prospective co-authors. To advance to the next stage, you have to pick one of them as your co-author, designating their collaboration as the chosen Turn 2. This action cannot be undone.

Choosing a co-author will move the Post to COLLABORATING status and grant your chosen co-author access to the Post's Dashboard page. In this status, the Post becomes private again (removed from public feeds), accessible only by you and your co-author. From here on, both you and your co-author have equal rights and the Write With Me platform does not distinguish anymore between the original Post creator and the chosen co-author.

The COLLABORATING status can last for as many turns as the co-authors need to complete their work. Ideally, turns should be alternated between the two co-authors, distributing contributions equally. But it is possible to Pass your current Turn to your co-author if you are feeling stuck, making it their Turn officially. The next Turn, which was supposed to be your co-author's, will then be yours, thus continuing the alternating pattern. A Turn once submitted by a co-author can be edited or deleted by that co-author only. Meaning, once a Turn is submitted by a co-author, only that co-author has full editing rights to that Turn.
Once they think its ready, both co-authors need to Publish in order for the Post to become publicly available again (under the Read tab in users' Home feeds). When one of the authors publishes, the Post moves to AWAITING_PUBLICATION status. There is in fact no functional difference between the COLLABORATING and the AWAITING_PUBLICATION statuses, except that the latter signals to the other author that the Post has been approved for publication by their co-author. Both co-authors can continue to add and edit Turns in this status. Once the other author has also approved publication by tapping on the Publish button, the Post officially moves to the PUBLISHED status. Even after fully publishing the Post, both co-authors can continue to add and edit Turns.
You can also write solo. To do so, on a fresh Turn 1 sheet, mark the solo box (next to the word counter) as checked. Tap the Create Solo Draft button to save the post in AWAITING_PUBLICATION status. Then from the post's Dashboard, Publish the post to make it publicly available when it is complete. Solo posts cannot be converted into collaborative posts.

Solo posts are also written in turns. You could just write it all in one turn. Or you could take advantage of turns to mark progress, or to break up the work into helpful chunks.
Solo posts can be a great way to showcase your prowess on the platform. When you are reading through collab-submissions on a Post of yours in INVITING_AUTHORS status, check out a prospective co-author's Bibliography tab in their Profile page to see if they have any solo posts.