Summary: If people are playing too fast, learn how to do a double pick on a banjo to keep up in this free video lesson from a professional banjo player.
"This next roll isn't really much of a roll. I call it the double pick. I use this when I'm playing with people that are playing either too fast for me to do a roll or if I'm trying to sing along to myself and I don't need to concentrate on my right hand too much. What happens here is similar to the alternating roll variation, is that your thumb moves from this string to this string mimicking a base. But instead of picking in between, with your, you know with your thumb and both of your fingers, you use these at the same time on these two strings. So it sounds something like this. A little faster it sounds like this. Now you can hear the difference between the alternating roll, and the double pick."
eHow Article: Banjo Picking: Double Pick