Save Group open state in localStorage, so upon browser refresh/page load, they can be opened in the same order.
The following example uses window.localStorage to preserve the state of open row groups. When the page is reloaded, the grid will access this and re-open the corresponding groups for you.
Server-side row model
Version 22: https://next.plnkr.co/edit/O6RxgdxsJwK0t3ft
Client-side row model:
Version 23: https://plnkr.co/edit/PEnSk8DJNexDY0Nd