Using certificates with SSH is the way to go for shared access servers. Here's an open source way (yes, I'm involved in the project) to manage authorization and access with asynchronous approvals:

https://github.com/cloudtools/ssh-cert-authority

https://github.com/smallstep/certificates

One nice advantage is its support for different provisioning flows. The oauth flavor allows you to hook into an existing identity provider to authenticate certificate requests.

Simply:

    $ step ssh login