according to Adding and Inviting Users , it is possible to allow people to sign up on Atlassian Ondemand:
Choose Invite Users on the dashboard, then choose User Signup Options.
Or take the longer route: Choose the cog icon at top right of the screen, then choose Confluence Admin. Then choose Users > User Signup Options.
Choose Allow people to sign up to create their account.
However, there appears to be no User Signup Options anymore. Was User Signup deliberately disabled in Atlassian Ondemand and the Documentation is out of date, or am I just missing something?