A user is anyone you want to have regular access to your Quatrix service - there's two types:
Pro users they can browse folders and share to any of your users or to their contacts who don't need a licence to download (normally your employees).
Associate users can only use your service to share files back to your Pro Users - great for external partners who need to regularly feed data into your organisation.
Both Pro and Associates use up one of your Quatrix licenses but Contacts(Personal/Site) don't need a licence to receive a file or send files back to you.
Owner is the top administrator of the account that has access to all Quatrix features and can purchase more users for the account.
Admin has the same rights as the account owner with the exception of tracking and paying invoices for the account.