A small change in the user interface should make it easier for Account Owners to add Team Members and Visitors to their Account Teams:

We separated out the buttons for adding Team Members and Visitors since there’s no cost to adding Visitors.

Adding people to an Account Team will enable them to view any boards owned by that account that are shared within the account: this is usually the default setting for new boards, since most folks want to make it easier for new account team members to catch up on what’s happening across the organization.

Individual boards can, of course, be made private.