While it makes sense to create new accounts in your organization, what do you do with all the other accounts you have now?
You can invite existing accounts to your organization, which means you can treat them just like a member account from an administrative point of view. This greatly simplifies the administrative overhead of your accounts, as there isn't a separate process for old and new accounts.
As this is generally performed once for each existing account, we will use the console.
All the AWS organizations functionality is available via the SDKs and AWS CLI tool. If you need to automate this process, you can.
