Group layers are a way to organize layers in your maps, by common geography, topics and themes, scales, for blending and effects, and more. Saving group layers as items was introduced with the October 2023 ArcGIS Online update and IMHO is a powerful and very welcome capability.
Use group layers to organize related layers in your map, optionally set exclusive visibility (showing only one specific layer without having to hide each of the other layers), then save group layers as items enabling reuse across your organization’s web maps or sharing outside your organization.
This blog article provides an overview: https://lnkd.in/dVFhJPDf
Living Atlas now also supports group layers, search the rapidly growing collection: https://lnkd.in/dtDyX9ZA