Add accountant in Accounting Settings now takes an email address or a handle and sends the same invitation as any other guest. Your accountant accepts it from the email or from Your teams, joins as a guest who can use Accounting and nothing else of the team, and is listed with your other guests on the Members page. Accountants count as guests, so any team can have them, Personal included.
An accountant opens your books by selecting your team in their Personal or team selector, where it is labelled Guest. The Books switcher at the top of Accounting is gone. Accountants you already had, and invitations not yet accepted, carry over as they are.