Hi Alicia,
Unfortunately, there is no way to share a folder at this moment. Insights use the same component that makes a folder in the user's content both in Portal or ArcGIS Online.
Folders are per user and a way to organize your own content. Groups will be an appropriate place for shared content. I understand the need for further organization within a group if the same group works on multiple projects. That said, this Idea can be repositioned to support searching/filtering groups and group categories within insights on the home page and insights workbooks
Thanks,
Akshay Harshe