Anywhere. They can read the first post, and think about a choice they think the main character might make, and write a story from there, contributing it as an option branch in the comments of that post. Or they can choose one of the option branches that are there, and see what option they think might come from that and continue that branch. I am not afraid of its getting out of hand. Upvotes can help the popular options rise to the top. And if someone wants to start their own book, I might contribute on that chain.
If a chain becomes uncohesive, I expect the last links won't have many contributors, so people backtrack to the part that makes sense and continue the chain from there.
Anarchy can rule.