Every item in ArcGIS Online has 3 options for sharing:
- To the public
- To your organization
- To one or more groups.
Groups are able to have members from other organizations.
Every item also has sharing permissions set independently (apps, web maps, layers, etc.) App can contain web maps and web maps can contain layers but they still maintain their individual sharing permissions. If you are sharing a web map to a group, you also have to share all the layers within that map to the same group as well to make sure everyone has access.