Update 2022: GitHub entitlements can help you define an IAM (Identity and Access Management) complete with audits.
2017: Note that since June 2017, you can apply an Admin access right to a sub-team (instead of one giant unique team as before)
See "Nested teams add depth to your team structure":

Child teams inherit their parent's access permissions, so repository permissions and @mentioning
among nested teams work from top to bottom.
If your team structure is Employees > Engineering > Application Engineering > Identity
, granting Engineering
write access to a repository means Application Engineering
and Identity
also get that access.
So that is another way to assign a user admin right: make him/her part of a sub-sub team called "admins" within an organization.
Official GitHub Documentation: Nested Teams.