Every logged in user can create a page on Arbital, but unless you are already a trusted in a domain, you have to submit the page for approval before it appears in the domain. This way a page can be reviewed by experienced authors before it's visible to the general public.

The process for submitting a page is simple:

Create a new page.
Publish it. (The page is now unlisted.)
Click "submit to math domain" button.
One of the reviewers will take a look. If the page can looks useful, they'll leave a comment. Otherwise, they'll approve the page.
Once you've submitted enough high quality pages, you'll become a member of the domain, and will be able to skip this approval process.

Submitting a page to a domain