Once you have purchased access to ArcGIS Online, by buying a Creator, Professional, or Professional Plus user type, you will be able to sign in to your ArcGIS Online account. An ArcGIS Online organization is a group of users from a company who can work together using ArcGIS apps and capabilities in a shared environment. After purchasing additional user types, you can then add your team to your organization from your ArcGIS account.
Please reference the invite and add members to your ArcGIS Online organization.