Group layers in Flex viewer

Hi,
we are three students who have to do some wprk in ArcGIS viewer for flex. We have some layers in ArcGIS Online added to viewer for flex and need to create "group layers" in Layer list. For example,,, we have railway layer, highway layer and road layer... we need to make group of these layers named "comunications". We don't have Arcgis server, so we dont know how to do it and the term of submission is on Saturday night. Please help us, we are helpless.