As the others said, his character really did not exist at all in GFI. But for the story's sake, I would say that he was a high ranking Soldier under Clemenza. In GFII Pentangeli has taken over since Clemenza's death, and has also taken over the compound back in New York ( remember Michael going to see him and commenting that he was very happy that the house went first to Clemenza and then Pentangeli). These events in GFII lead me to believe, again for the storyline's sake, that Pentangeli was right under Clemenza.
Don Cardi