Two identifications dominate and they point in opposite directions.
The first is the Mandaeans of the lower Tigris and Euphrates, a gnostic community whose religion turns on repeated immersion in flowing water and who revere John the Baptist. The argument in their favour is largely linguistic: the Arabic root behind the name carries the sense of immersing or dipping, and a community organised around baptism fits it directly. Their neighbours have called them by a related name for centuries, and they hold protected status in Iraq and Iran on that basis to this day.
The second is the people of Harran in upper Mesopotamia, who maintained a planetary and astral cult into the Islamic period and produced significant scholars and translators. The account preserved in the Fihrist of Ibn al-Nadim reports that the caliph al-Mamun, passing through the region, told them they must become Muslims or People of the Book, and that they adopted the Sabian name in order to qualify. If the story is accurate, the Harranians are Sabians by a legal manoeuvre rather than by origin, which does not settle who the Quran meant.
Other proposals have named Elchasaite baptists, Manichaeans, and a community that has simply not survived. The atlas records the question as open. It is a matter for Semitic philology, for the archaeology of upper Mesopotamia and for the study of the early Islamic legal sources together, and no one discipline can close it alone.