Hi Paul,
We hope to have an Explorer release later this year, but unclear at this point whether or not the Group Layers in MMPK will be included. The enhancement to support Group layers in the Mobile Map Package will be in various places throughout the platform. Currently, the group layer is not in the Mobile Map specification. When it is (roadmap tbd), various pieces (including ArcGIS Pro and the ArcGIS Runtime SDK that Explorer) will need to be enhanced to consume this.
Thanks,
Heather