Great question! Here is commentary on the people of "Rass" by Syed Maududi in his Tafhim al-Qur'an:
Before this the people of Rass (Ashab ar-Rass) have been mentioned in Surah Al-Furqan: 38 above, and now here, but at both places they have been only referred to as of those nations who rejected the Prophets. No other detail about them has been given. In the traditions of Arabia two places are well known by the name of ar-Rass, one in the Najd and the other in northern Hejaz. Of these ar-Rass of the Najd is better known and has been referred to more often in the pre-Islamic poetry of Arabia. Now it is difficult to determine which of these was the home of the Ashab ar-Rass. No reliable detail of their story is found in any tradition either. The most that one can say with certainty is that it was a nation which had thrown its Prophet into a well. But from the mere allusion that has been made to them in the Qur'an one is led to think that in the time of the revelation of the Qur'an the Arabs were generally aware of this nation and its history, but the traditions about them could not be preserved in historic records.
In the commentary on Furqan:38, he says:
There is no definite knowledge about "the people of the Raas'' . Different commentators have said different things about them, but nothing is convincing. The only thing that may be said about them is that they were a people who had killed their Prophet by throwing him into or hanging him down a "Rass " (an old or dry well).
Ibn Kathir in his tafsir on Furqan:38 says:
As for the Dwellers of Ar-Rass, Ibn Jurayj narrated from Ibn `Abbas about the Dwellers of Ar-Rass that they were the people of one of the villages of Thamud. Ath-Thawri narrated from Abu Bukayr from `Ikrimah that Ar-Rass was a well where they buried (Rassu) their Prophet.