Does the layer still appear in the Layers panel when you and / or other users in the group open the map? Is it when the map is opened in ArcGIS Explorer Online that the layer cannot be seen, or is this when opened in the map viewer part of the website, or in another web mapping application maybe? Perhaps you can share the map publically, or if you invite me to your group (username sgill) I will open the map and see if I can find out the problem.