Depending on your payment plan, you may or may not need to access your Cloudasta billing portal to increase the number of licenses before creating new users. Please review each one of the scenarios below:
How do I add more User Licenses (Annual Commitment Plans)?
How do I add more User Licenses (Flexible Plans)?
Once you have enough licenses, please go to your Admin console (admin.google.com) and navigate to your 'Users' page to click in the option 'Add new user'.