Access to one of the member account in control tower from another client external AWS account

If you don't want the account to be part of the organization, then you'll need to grant access. To grant access to an external account, you can treat it like a third party account. This will involve granting a trust as well as permissions. For extra security consider adding an external id which is like a key or password.

https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user_externalid.html

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