the ModelGrouper operator behaves like every operator in RapidMiner: It uses what it recieves. You might make a breakpoint before the ModelGrouper. The result pane will show, which Models are delivered. These are then combined in one single model. The order of the models is determined by the order they are recieved (and hence they are represented in the tab order in the result pane).
These models are then treated as one model, so that all of them are written into a single file, if you use the ModelWriter. They are all restored if you load them again, indepentendly from any operator writing/loading before.