It should be possible for someone to be added to a group but not participate in (automatic) sharing. For example my wife would join the group, see the bills, possibly adding new ones, but when split equally or with percentage I would take her part hence saving us for owing money to each other. An observer flag would fit well, possibly combinable with pre-saved percentage per group member. In this case she would get 0%.
It should be possible for someone to be added to a group but not participate in (automatic) sharing. For example my wife would join the group, see the bills, possibly adding new ones, but when split equally or with percentage I would take her part hence saving us for owing money to each other. An observer flag would fit well, possibly combinable with pre-saved percentage per group member. In this case she would get 0%.