I forgot to ask this during my lab session, but I noticed iPadOS is not listed under supported OSes under the GroupActivities documentation page.
iPadOS supports FaceTime, but is it that GroupActivies doesn't work on iPadOS? This would be a crying shame since one of the examples specifically involved drawing collaboratively. The iPad is the perfect device for that use case.
EDIT: Quick edit. Coordinate media experiences with Group Activities mentions iPadOS support, in which case the first page I linked might have a missing OS entry.