You can signup and create a new Atlas workspace from Atlassian's website. You can either:
Create a new site with Atlas, or;
Add Atlas to an existing site
If the person adding Atlas to an existing site is not a site admin, admins will be notified and will need to approve the request to add Atlas to the site.
Note, you can have one Atlas workspace per Atlassian site, and many Atlassian sites per Atlassian organization.
See here for more information about the relationship between and managing sites and product workspaces in your Atlassian organization.