Please see Calvin or Shemeir's responses under comments. Both are good alternatives. You could also say: They/The group had been (very) problematic. They/The group had caused a lot of trouble.

I hate to say it but neither one is correct. I am not sure what you are trying to say but I'll take a guess. "Those people have been causing a lot of problems." or "Those people have had a lot of problems."

I would suggest using "trouble" instead of "problem".I think the best way to say it would be: Those people had caused a lot of trouble. Those people had been a lot of trouble.