Giving your accountant access
Invite your accountant the same way you would anyone else, and give them the Viewer role. They see the books as they stand, which is better for both of you than a spreadsheet emailed at the year end and out of date by the time it lands.
Use the Viewer role
If they need to make changes
Some practices would rather post adjustments themselves than send you a list. If that is the arrangement, give them Admin instead and be clear with yourself about what that means: Admin is the same access an owner has, short of nothing.
Ending the engagement
Remove them and access stops at once. Everything they did stays on the record, because an audit trail with people deleted out of it is not an audit trail.
If your accountant would rather run several clients from one place, they can join the partner programme instead of being invited company by company.