Can you check the following setting:
admin console --> people --> settings --> registration settings --> New Account Creation Enabled
If it's false, set it to true and I'm guessing the problem goes away.
If it's already true:
what version of Clearspace are you using?
do you have any other users (besides the admin user) created?
what database are you using?
have you created / applied any permissions?
Thanks for the response AJ.
The clearspace version is 1.1.0 and the problem was:
System -> Management -> System Properties -> jive.auth.disallowGuest = true
Changing the value to false solved the issue.