While settings up your Spotinst account you are creating an Organization and Account entities. The Organization will be singular and you can create multiple accounts for different teams or projects within the Organization. Each account can be set up to use any supported cloud provider and cloud provider account.
To add users to an account please open the Settings
At the Organization level, you can add a user to the default account of the organization - this means the user will only be added the default account with the "Role" you define. This user will also enter a "pool" of organization users. Any user can also be added to any other account via the Account Tab user menu as listed below.
At the Account level - where you can add a user to the specific account you are currently logged into - this means the user will be added to the current account with the role defined while creating the User in the Organization tab.