OnDemand users, in particular those that have their OnDemand instance linked to Google Apps may occasionally get the following error when logging in to Atlassian OnDemand for the first time:

Failed to login to Atlassian OnDemand
Reason: Unable to grant access to CONFLUENCE to new user.
Please contact your Atlassian OnDemand administrator if this error persists 

This is a common error when you have used up your licenses for your instance. What happens, is new users are added to Google Apps then synced automatically to Atlassian OnDemand. At this point no license has been assigned to the new user. But upon first login, it attempts to assign that user a license, which fails because there are none available.

All you need to do is login to http://my.atlassian.com/ and add licenses to your account to cover any new users you have added.