As for sentence A - you might look at it as if you were trying to put emphasis on the fact that self-help groups were very common all over America in the recent past, are still and may be so in the future. You treat the present moment as the point of reference - you refer to a situation having its start in the past leading up to the present moment.
Sentence B simply tells us about the fact of self-help groups becoming very common all over America. It's a matter of emphasis, to my mind.
- For Teachers