Hi @RoseZhou2,
> Is this feature available in Portal for ArcGIS 10.8.1?
I believe you're asking if group layers are available in an enterprise portal at 10.8.1. The answer is "no". Group layer capability was added to AGOL in July 2020, and ArcGIS Enterprise 10.8.1 was released in July 2020 - typically, functionality introduced in AGOL is added to Enterprise later on.
Hope this helps,