There are a couple of ways to achieve what you're looking for. Depending on your use case, you can either make the calendars public (Default is AvailabilityOnly, you can change this to LimitedDetails instead) and whoever interested can add these calendars manually to their Outlook (as an additional calendar). Or, as you've asked, you can grant the permission in bulk using the Add-MailboxFolderPermission cmdlet.
Ex: Add-MailboxFolderPermission -Identity email@example.com:\Calendar -User firstname.lastname@example.org -AccessRights Reviewer
[This will grant Laura reviewer permission over Ayla's calendar]
You can run this against each account that you want to grant the permission to. Replace "Add" with "remove" if you wish to remove a permission that was granted (suffix the cmdlet with "-Confirm: $False" if you don't want to be prompted each time when applying the permission).