Simple! Just click on People then Groups. From there, click on the name of the group that you want to assign a manager to.
From there, you can either choose an existing manager or create a new one by navigating to the Managers tab.
Once you have assigned the user as a Manager of that group, you must select which permission levels to assign to them. To do this, click on the Permissions box:
Select the different permission levels that you want assigned to the Manager of this Group: