Hey there,
Yes you can list out all users attached to a dev-id by hitting the /subscriptions endpoint which will return a list of all users.
You can also use the endpoint /userInfo with a reference_id
query parameter to list out all users (devices) attached to given reference that you may supply when authenticating.
Also, if you wish to just see all the users on your account: navigate to your Dashboard > Tools > Debug > Users and view the users table which will show you all users on your account.