Once you've followed these instructions to switch on the Parent Portal, any guardians would be able to log in (providing they know how to get to the login page). You don't need to update access in bulk or individually, as this is automatically applied.
However, they won't be automatically notified by email, SMS or any other method that they can log in. To let parents know they can log in, you need to manually send them a welcome email: Sending and resending login details to Guardians
Because Arbor works in this way, you can't switch on the Parent Portal and Arbor staff for only one or a few guardians. However, you can select a specific guardians to send the welcome email to, have them log in, then send out the login details to the rest of your guardians to give them access in bulk.