I have some less files in the main Site Theme. Is there a way to reference them within a Group Theme HeadScript?
This is not supported. Because of the sensitivity of LESS syntax, referencing/including a file from one versionable component in another could easily result in errors.
Instead, it is recommended to either define a reusable style in the parent theme or define configuration values that can be read within the child theme. A child theme (or widget) can read configuration from the parent theme via reverse inheritance: https://community.telligent.com/community/11/w/developer-training/65592/theme-inheritance#LESS_CSS_Reverse_Inheritance
