I'm not really sure where to start here. Perhaps with a bit on my experiences as a "hedge keeper," although we were more likely to call it being a guardian back in those genero-pagan days.
I have acted as a boundary guardian for women's circles, keeping people from accidentally intruding on a ritual and allowing women who would have had to do this task to attend the ritual itself. Although this description sounds rather mild, it was a serious role to me, more akin to warrior work than a simple usher role. Although in these particular cases, trouble or deliberate interference was unlikely, I was always prepared to be a physical defender.
It's a point of honor for me to never eavesdrop on a ritual which might be within earshot and never let my conscious mind retain anything I did hear. To me, that was part of being a guardian, the ability to maintain trust by clearly focusing on the role.
In a less spiritual setting, I have also been a "peacekeeper" for Gay and Lesbian marches. In recent years, in large cities, the danger is rather minimal for people serving in this capacity. When I first did this in 1984(?) in Northhampton, MA, only a few hundred people participated in the march and there were at least fifteen or twenty rather aggressive hecklers. I was spat on and taunted by several of these young men. I had to be calm and alert yet be prepared for the very real possibility of violence.
So these are some initial thoughts on the role of hedge keeper.