Our Sages derived many laws pertaining to prayer from the incident recorded in the beginning of the book of Shmuel regarding Chana, mother of Shmuel Ha'Navi, who went to the Mishkan (Tabernacle) in Shilo in order to pray to Hashem that she be able to bear children. Through the power of her prayer, he holy son, Shmuel Ha'Navi, was born.
The Prohibition to Sit in Close Proximity of One Praying
Our Sages (Berachot 31b) learned from what Chana told Eli Ha'Kohen while she was praying, "I am the woman standing with you here to pray to Hashem," that one may not sit within four Amot of one praying Amida. Four Amot is equal to approximately 6.5 feet.
There is no distinction whether one wishes to sit in front or on the side of one praying, for as long as one is within four Amot of one praying, one may not sit there; rather, one should move slightly more than four Amot of the individual praying, at which point one may be seated. Clearly, there is no difference whether the individual praying is a man or woman and the prohibition to sit within their four Amot remains the same.
The Reason for this Prohibition
Our Sages disagree regarding the reason why sitting within four Amot of one praying is forbidden. The Ba'al Halachot Gedolot writes that the reason for this is because Hashem's presence rests within four Amot of one praying and sitting there is therefore disrespectful. It seems that according to the holy Zohar, the reason is likewise due to respect for the presence of Hashem.
The Tur writes that the reason for this prohibition is because it seems that while one individual is accepting the sovereignty of Hashem upon himself, this individual is sitting idly and complacently and not accepting Hashem's kingship.
The Meiri writes that the reason for this prohibition is in order for the concentration of the one praying not to be disturbed by those around him. The Sefer Ha'Michtam adds that the reason is in order for the individual praying not to be ashamed by the fact that the person sitting next to him sees him crying while he is praying.
Sitting Behind One Praying
Regarding sitting behind one praying, this would be contingent on the aforementioned reasons. If the reason why it is forbidden to sit close to one praying is out of reverence for the presence of Hashem, this should be forbidden even when one sits behind one who is praying. However, if the reason is for the one praying not to be ashamed or disturbed, these reasons no longer apply when one sits behind the individual praying.
Halachically speaking, the Rambam (Chapter 5 of Hilchot Tefillah) writes that it is forbidden to sit "on the side of one praying." It would seem from here that one may sit behind one praying. Similarly, Maran Ha'Shulchan Aruch rules that one may not sit within four Amot of one praying, "whether in front of him or on his side." It would likewise seem from here that sitting behind him would be permissible. Indeed, some great Acharonim write that according to Maran Ha'Shulchan Aruch, one may sit behind one praying. Nevertheless, there are those who rule stringently on this matter. Thus, if one acts stringently and does not sit behind one praying, he shall surely be blessed. According to the Ashkenazi custom, sitting behind one praying is certainly forbidden, for this is how the Rama rules and Ashkenazim have accepted the Rama's rulings.
Sitting in Close Proximity of the Chazzan
Based on the above, the same would apply to one who wishes to sit within four Amot of a Chazzan while he recites the repetition of the Amida and one will not be permitted to do so until he distances himself more than four Amot away from where the Chazzan is standing. Maran Rabbeinu zt"l points this out in his Responsa Yechave Da'at (Chapter 5) and writes that even those who act leniently and sit during the Chazzan's repetition must make certain not to do so within four Amot of the Chazzan.
Summary: One may not sit within four Amot (6.5 feet) of one praying Amida, whether one is sitting in front of him or to his side. Some rule stringently regarding sitting behind one praying, as well. It is likewise forbidden to sit within four Amot of a Chazzan while he is reciting the repetition of the Amida.
