The attached zipped solution file includes an example of a Group Approval stage.
When each reviewer approves the folder, his/her name is removed from the To Do list. Each time a name is removed a conditional action checks to see if the To Do list is empty (i.e., if all reviews are completed). When the To Do list is empty, the conditional action is triggered and the folder moves to the next stage in the process.
The key configuration points:
1. The Approve (or, in my demo "Yes") loop-back action should be set to "Remove from to-do list"
2. The conditional action should have its Only Start Action If property set to:
(GetToDoList(ProcessContext.FolderId)==ListItems(null))