Every Muslim is required to offer the Fard part of his Prayers in congregation. Mosques are built for the purpose of congregational Prayers. A Hadith tells us that the reward of a Prayer offered in congregation is twenty-seven times more than that offered individually.
The Imam should be chosen by the congregation, keeping in mind that to the best of their knowledge, he is a pious person and the most well-versed among them in the Holy Quran. The Imam can also be appointed if there is a regular religious authority, e.g., a Khalifah. Whoever is chosen or appointed as Imam must be followed in the Prayer even if somebody thinks that the appointed person is not worthy of it. For those people who doubt the worthiness of an Imam, the following instructions of the Holy Prophet(sa) should suffice:
Transliteration:
‘An Abi Hurairah qala Qala Rasulullahi sallallahu ‘alaihi wa sallam: Assalatul maktubatu Wajibatun khalfa kulli Muslimin, barran kana au fajiran wa in ‘amilal kaba’ir. (Sunan Abu Dawud, aljuz’ul-awwal, Kitabus-Salat)
Translation:
Hadrat Abu Hurairah(ra) related that the Holy Prophet(sa) stated: To offer Fard Prayer behind any Muslim Imam is essential, whether he is a pious person or a sinner, even if he had committed a grievous sin.
If, at the prescribed time for any Prayer, two or more men are present away from a mosque, they are required to observe the prayer in congregation. When only two males come together for Prayers, one of them should lead the Prayer. They should stand together so that the 2nd person stands on the right side of the Imam.
If a man performs his Prayer at home and a female member of the family like to join him, then, she should stand on his left. In the case where two men are already offering their prayer in congregation, and worshippers arrive, they should arrange themselves in a row so that the Imam remains in the middle. In normal cases when three or more persons offer their prayer in congregation, the Imam should stand in front, all facing towards the Ka‘bah. In exceptional cases however, the Imam can also stand in the middle of the first row along with other worshippers.
According to some schools of thought, there should be at least two persons in the last row behind the Imam. These schools of jurisprudence even permit gently pulling someone from the last row so as to form a new row with two persons instead of one.
Other schools discourage this act, as it not only causes disturbance to the person who is gently pulled but also to the others who are already engaged in prayer.