Normal for cat to nibble a bit during grooming?

One of out kitties nibbles on her fur and skin a bit when she gets really into grooming. Not casual grooming, it's only when she's really into it. Tonight, she was grooming our other kitty and I noticed she did it to the other cat as well... the other cat didn't seem to notice (it was behind her ear so she probably didn't get any skin anyway).... but it struck me as kind of odd- maybe I just hadn't noticed it before? Not aggressive, right?

Stan does this too when he really gets going, but I've never seen him do it to Bella. This time of year he's pulling out tufts of hair in spite of frequent brushing and time with the furminator. I have tumbleweeds of Stanley hair all over my hardwoods...

my friend's cat nibble the front of her feat when she's getting brushed or really getting into being pet

Yes, that's normal. They do it to remove knots or hard particles stuck in their fur. When my cat "grooms" my hand, he tries to pull my ring up with his teeth, which cracks me up. To him, this hard knobby thing is "dirty," and he's trying to clean it off of me.