Agree, Groups have no option to isolate managers so you're only option is one that needs to pull from the additional fields...May be easier if you are able to identify your managers as part of the profile creation process...how do you provision your users?

That at least confirms my impression of the situation. Users are provisioned automatically once they’re added to our HR system - so basically there’s no real way to intercept and add anything like that.

I’ve added the idea now too: Make 'Managers Permissions' a field that can be used for groups / enrolments. | Community (docebo.com)

We do this as well and have created a field that captures their employment type so we can use that to form groups. You can adjust your import source to capture something similar and this would automate that part of the process for you. Depending on what method you use to provision your users (we use several methods, a custom connector and the docebo automation tool) the file you use to import data can transform some of the data for import to additional fields in Docebo; this way you don't need to do anything more and your managers are identified.