I only see an option to add an individual activity, not a group activity, from the Scouting app (version 126.96.36.1992). Where are you seeing an option to add group activities?
Scoutbook itself no longer actually has the activity logs. Entries now have to be entered through the IA2 interface (advancements.scouting.org), although the link from Scoutbook (scoutbook.scouting.org) should bring you to the same interface as going through IA2.
I’m not seeing an issue adding individual activities through IA2. I just added an individual hike for my son, while logged in using my parent credentials under the fleur-de-lis pulldown menu in the top right corner. What’s the error that you’re getting via the Scoutbook/IA2 interface?