Same two characters as 分子, the molecule, but the first syllable is fourth tone here. If the sentence is about people rather than chemistry or maths, the reading is 分子.
usage
This reading almost never stands alone. What comes in front names the group, and most of the set combinations are labels for people, several of them accusations, as in 犯罪分子, a criminal. Used about someone, it sounds official rather than neutral.
