A group just outputs its “global” state change but not every single sensor change within the group.
Means for example if the state of one of the sensors changes from false to true, the global state of the group changes to true. if another sensor within the group changes to true while the group is still true, there wont be any event for that.
So it would be nice to have an option to output every state_change within the group.
I was researching something else and remembered your post when I saw this. Not sure if you have tried. If it works at all, probably not the optimal solution, however it does appear that you can enumerate through the members of a group:
WORKING WITH GROUPS
The expand function and filter can be used to sort entities and expand groups. It outputs a sorted array of entities with no duplicates.
https://www.home-assistant.io/docs/configuration/templating/#working-with-groups
I’m pretty sure this can be achieved with a template, but it is beyond my meagre programming abilities. I solved this by using a boolean helper. Automation toggles helper whenever any of the listed motion sensors trigger (5 in my case). This boolean can be used as a trigger in automations. I think a different helper could be used to count motion triggers, but I never tried it.