I enclosed one simple sample based on your scenario, to maintain or restore expand and collapse grouping state on WebGrid with paging enabled.

The idea is quite simple. I added a global variable as the variable to keep/store the expanded group nodes. In OnGroupExpand and OnGroupCollapse client-side event, information like current page index; row position; and expand/collapse state of the group node are obtained. These information are processed and being assigned to the global variable as an array.

Next, we can utilize the OnAfterResponseProcess client-side event to retrieve the list of expanded nodes and expand the nodes that previously expanded. The OnAfterResponseProcess is the client-side event that specifies the client-side (Javascript) function that will be invoked after the response action of a request has been performed.

For more detail information, please have attached sample tested on your end and let us hear whether it helps or not.

*The WebGrid in this sample is bind to Customers table of Northwind.mdb data base using AccessDataSource control. The data source file is available inside the App_Data folder WebGridSamples project or inside this folder, $ispct;ProgramFiles$ispct;\Intersoft Solutions\WebUI Studio for ASP.NET\Data.