1- Before execute tests, maybe in the before() method, select a random user (loaded by fixtures), execute a login and storage an accesskey.
for general before and after helpers you can use before and after annotations and as you mentioned you can use your stored user fixtures to load randomly user which is supported by faker lib
2- Execute the tests to the nodes with the saved access_key.
_You can store your generated auth token || access key in $var and make it global $var to access it from anywhere _
Hope this answer your question!