At the current state it's only single-user, but I'm working on a multi-user implementation. It should be available as soon as I have a proper implementation for the ActivityPub Group actor (similar to what WriteFreely already does, where you have a Group @blog actor that aggregates post from multiple Person actors). That would also unlock full compatibility with Lemmy/NodeBB and other threadiverse implementations. But it takes a while to get done right - and a multi-user set up is also a design challenge when your selling point is to have a blog running on a single folder with Markdown files.
About a free instance, it's not currently available (you'll need your own domain and host and then you can just spin up a Docker container), but AFAIK YunoHost is considering adding it to their offering.
Initially Webmentions and AP reactions were rendered on separate panels (with the Jinja rendering templates provided by the
webmentionsandpubbylibraries respectively), but then I found that a bit confusing and cluttering, because there were basically two comment sections with two different time-sorted lists of threads. But if people prefer the other way around I could also add it as an option or a filter toggle ๐