@AlexaDavis -- Collaborative editing is not available with a free, public ArcGIS Online account; it requires a subscription account.
Non-profit organizations can get special pricing for subscriptions through the Esri Non-Profit Program which you can find out more about here: Nonprofit Software Pricing Program (esri.com). If the organization you are working with requires collaboration at this level, I would recommend they look into establishing an organizational subscription account through this program.
Owen Evans
Lead Product Engineer | StoryMaps