This article explains best practices around on campus employment, divisions, and add Hiring Managers to the On-Campus Employer accounts.
The way you setup hiring managers ultimately depends on how involved they need to be in the job posting process. The first question to answer is if you are going to continue posting all positions on behalf of the divisions and if they need to see the emails, or are they going to be going into the system to post jobs themselves?
If you or another contact will be posting all positions on behalf of Hiring Managers:
We recommend keeping these Hiring Managers as contacts, and just adding them as a contact on the job with the correct notification cadence. This will only send them emails with the application packages and they won't have to be in the system.
If Hiring Managers are going to be posting for themselves:
We recommend pre-configuring all of those divisions/departments ahead of time in your on-campus employer account. Then we recommend navigating to your "Manage Staff" page in the on-campus employer account and sending those hiring Managers an "invite link"
This will populate a window that enables you to share this link. Any staff member that uses this link to register will automatically be connected to the employer account for your school. Here's an example of what this invite looks like:
This article has more background on on-campus employment and canned language that you can use if you want to send a mass email to these contacts.