In this configuration you can define ssh keys for different users. By clicking the plus button you can add users and then you can add the appropriate ssh keys. This is again defined for the specific group or device. So there is a very granular way for defining which ssh keys should be available on which device or group. Defining them for all devices will propagate them throughout the fleet (assuming that all groups inherit). Please keep in mind that assigning a new ssh key to a user will overwrite any previously assigned keys, so ensure that all necessary ssh keys are set for each user.
Here is how you create an ssh key
This tutorial explains how to create a public-private ssh key pair on your machine.