iOS Certificate Signing

A few resources, to start:

Hello Mike, 
My name is Hammy and I am a senior Advisor with Apple Developer Program Support. I am following up with you regarding your inquiry. 
Please keep in mind that Apple Developer Program support team provides administrative support for developers. That said, I have found the following resources on code signing you may find helpful:
What is app signing?
Create, export, and delete signing certificates
If a code signing error occurs

Additionally, as an individual member, while you can not grant access to your team to access the program assets ( including identifiers, provisioning profiles, and certificates ), you may distribute the beta version of your app using either ad-hoc distribution, or TestFlight to your testers. Here are few resources you may find helpful:
Distribute to registered devices (iOS, tvOS, watchOS) 
Beta Testing Made Simple with TestFlight
Distribute an app using TestFlight (iOS, tvOS, watchOS)

If you have any further question, please do not hesitate to respond to this email. Your case number is 101420747342. 
HammyApple Inc.