To be able to add additional users to your organization, you have to be configured as owner or manager in the support system, if you need this to be updated please create a support ticket and the support team will enable you to do this.
You can log in to your account at help.filecloud.com.
Click on your user name drop-down on the top right corner and click on Organization.
In the organization mode, you will find a button, you can add a user by clicking the "Add User" button and filling in all your user information:
Note: You can set the user organization access level by choosing the option at the end of the fields before creating the user. This can be changed later.