1) Set a temporary group membership on a user to expire in the near future (an hour)
2) Let the time pass and verify that the user was in fact removed from the group
3) Examine the the change history of both the group and the user and see what attribute(s) were modified by the expiration
4) Create an ActiveRoles workflow that reacts to that attribute change. This could be tricky because it might be difficult to distinguish the change made my the temporal group membership change from a "regular" one. One clue might be the fact that it is the AR service account making the change though there might be other AR processes in your environment that do this as well.