FastMail Vault (business) & FastMail Loop (family) allows an email administrator to group together a number of accounts under a single entity and allows easy management and control of those accounts. The administrator can easily add, delete, and manage the user accounts from a simple administration screen at any time.
Each account in a business or family retains its own standard features, such as the individual access through the web, IMAP, or POP and the ability to send messages via the web or SMTP. Each account has its own mailbox, address book and file storage area.
On top of this, the business/family has the ability to share emails and files on a per-folder basis, and addresses via a global address book.
Accounts within a business/family can be created in a domain your own (eg email@example.com), or you can can create accounts in one of the FastMail domains (eg firstname.lastname@example.org).
If you add users during a billing period, you'll be charged pro-rata amount to the end of the current billing period for the extra user. If you delete a user, you'll receive a pro-rata refund into your account pool for future use.
You can group together operations to keep actual credit card billing events to a minimum. For instance, you can add multiple people, delete some, and upgrade/downgrade others all in one screen and only be charged one amount to the credit card for creating/modifying all those users. For instance, if you delete 1 user, and add 4 users, you'll only be charged the difference of adding 3 new users.
When you create a business/family, there is a special master user created at the same time that is attached to the business/family. This account is special. It's always an administrator of the business/family and stored business/family related information like the credit card for charging, the global address book, any domains/aliases for the business/family, etc. It isn't designed for general usage, it only has a small storage quota.
The master account is mostly just a holding and bootstrapping account. In general the idea is that once you've signed up a business/family and started in the master account, you then create your own account, and accounts for employees/family members.
You then set your own account as an "administrator" account as well. In the future, you can do all the administration from your personal account rather than having to login to the master account, which just becomes a place holder and fallback account.