Account sharing allows multiple members to share access to and control over a single account. This includes actions like making deposits, renewing domains, and editing websites. It is significantly more powerful than adjunct access.
Account sharing is primarily useful for organizations, where more than one person is involved in the day to day operation of its hosting related services, or even where the one person who is usually involved in those day to day operations has the temerity to get sick or go on vacation.
It can also be useful in cases where a non-expert wants to operate a site with the help of an expert. They can share the account so the expert can adjust DNS settings and manage domain registration as necessary to support the site. It likewise supports setups where one person is paying for things and a different person is doing them.
Account sharing is very powerful, and you should always be very careful that you trust any person with whom you share your account. They essentially become co-owners of the account, with full privileges to do whatever they want with it. That includes, but isn't limited to, transferring funds and assets to accounts of their own, deleting things, or transferring registered domains to other providers.
To use account sharing, each person must have their own membership, which provides them with their own personal username and password. (Remember: accounts and memberships are different, and you can have multiple accounts, but not multiple memberships).
Once each additional person has set up their own membership, then you can share an account with them from the Account Information panel for that account. Just have them give you their login name and add it to the list of "Members Sharing This Account." To remove another member's shared access, click the "Unshare" button next to their name on the list.