how do i share a folder in icloud with a family member

It is the same as if you weren’t in Family Sharing. See: Share files and folders in iCloud Drive on iCloud.com - Apple Support and Use iCloud File Sharing to share folders and documents with others - Apple Support

Also note the following about sharing an iCloud storage plan, from Share iCloud+ with your family - Apple Support

When you share an iCloud+ subscription, everyone in the family group uses their own account. That way your photos, documents, and other information stay private, even though you're sharing access to the service. You simply share the features and space available in iCloud+ with your family members, so you only have one plan to manage.