The first time I encountered the "bell humpers" as you so eloquently call them, I defeated a single invader only to have two more appear the instant he was gone. I managed to kick of on them off the top of the ladder leading to the top of the tower only to have the other one step over his body and gang up on me with the little fuckers running around up there. The second time I encountered them was right away as I was headed back to get my souls. I was invaded the instant I stepped into the belfry and was hammered with greater soul arrows until I got thrown off the bridge at the entrance.