After enabling the Dependabot Security Alerts you need to explicitly grant access to alerts in the Security & Analysis settings (https://github.com/[org]/[repository]/settings/security_analysis). By default collaborators don't see the Security "tab" unless they have admin rights to the repository (which we don't use).
Is there a way to do this with the GitHub API or another automated/scriptable way? This doesn't seem to be in the API, and it's not very practical to do manually with 100's of repositories.