GitHub Enterprise Serverのユーザ名あるいはメールアドレスを使えば、誰でも Organization のメンバーにすることができます。
Managing an individual's access to an organization repository
You can manage a person's access to a repository owned by your organization.
Reviewing the audit log for your organization
The audit log allows organization admins to quickly review the actions performed by members of your organization. It includes details such as who performed the action, what the action was, and when it was performed.