There is no public creation path
This box has one operator, and Sites are created by them — either on the box itself, or for a friend who was handed a single-use invite code. There's no "create account" button anywhere on rentfree.site, because there's no account: possession of your Admin Key is what makes a Site yours.
If you were given an invite code
You'll need an SSH keypair. If you don't already have one:
ssh-keygen -t ed25519 -f ~/.ssh/rentfree -N ""This creates two files: rentfree (private — keep it) and rentfree.pub (public — this is what you send).
The operator's invite code and your public key together are all signup
needs. There's no email, no password, and the server never sees or
generates your private key — it only ever receives the
.pub file's contents.
What you get
A name of the form yourname.rentfree.site, bound to the
public key you supplied. From here, everything is deploying.