The Sabians were followers of John the Baptist. Quite an interesting group. In some ways it can be argued that Isa(as) was a Sabian. That denomination still exists and has a handfull of followers left in Iraq. However, The word Sabian refers to 2 different things.
Be Cautious if you are using it as it is used in the Qur'an. Several translators, including Pikthall, confused se'en with Sh'een resulting in Pickthall translating it into a different group then most scholars believe is meant. Sabians and Sabaeens are 2 totaly different things.
Brother Fi gave an excellent link as to what is truly meant. In reference to the Qur'an that is what is meant.
Sabians practice initiation through submersion; intended to hark back to the innundation of the world during the deluge of the time of Noah which cleansed man's sinful nature from the face of the earth. Hence ablution is an important part of the Sabian religions. The practice involves falling back into running water in condemnation of the sinful nature and is a sign of submission and awe before God. There has been much speculation as to the origins of the religious endonym from this practice. Some have argued that the term Sābi'ūn derives from the Syriac root S-b-' , referring to conversion through submersion; the Syriac (and Hebrew) nouns derived from this root refer to proselytes, both "Judaisers" - non-converts who followed certain basic rules of Judaism - and early Christian converts of non-Jewish origin and practice. These latter were called Theosebeians "God-Fearers", Sebomenoi "Believers", or Phobeomenoi "Pious ones" in Greek sources (from the root meaning "to fall back"). The Greek etymology for sebomai, applied to the proselytes, is in the word eusebeian meaning a kind of godliness and reverance or worshipfulness. However, proselytization has also long been associated with submersion and although the Greek etymology of the Sabian appellation is more than likely the original (there being no record of any similar people prior to the Hellenic era), Mandaean Nasaraeans claimed the word Sabium (from Subi or Sabi, plural Subba or Sabba), colloquial Mandaic or Syriac in origin, for themselves giving the meaning "to submerse" or "plunge in" .
According to Islamic scholars, the word Sābi'ūna (Sabian) is derived from the verb saba’a, which refers to the action of leaving one religion and entering another .
Tabari said: as-Sābi'ūn is the plural of Sābi', which means "proselyte" (such as an apostate from Islam) who has left his original religion, or anyone who has left the religion that he used to follow and joins another. The Arabs called such a person Sābi'. Though meaning upright and monotheistic these days, the word Hunafa` also derives from the Syriac word "hanifo" which literally means Agnostic. Similar distortion has occurred with the word Mushrik which though meaning polytheist these days originally referred only to ****tuf/shirk or the act of establishing a parter with God -most commonly in the form of an evil opponent.