Team and hierarchy controled ownership

On account and contact details you can assign a owner, the owner can be one person, a team of people, or a system group for example Administrators

With the owner assignment you can choose which users has access to various accounts. If everyone work with the same accounts it's easier to setup access for all users.

In the user tab you can set a specific user as the owner, if multiple people are involved using the team tab could prove more efficient. Create a team for the appointed users and only they will have access.

In the system tab you can setup access based on the different user groups, meaning if you have chosen Account managers they'll have access to all the accounts.

Ownership with hierarchy means that if you're above someone on the hierarchy you'll be able to see which account and contacts they own but they won't have access to the ones you own, unless you setup permission for it.

To setup teams and users that you can sign ownership to go to user/team manager, you can read more about that here.