It is a sunnah to offer supererogatory prayers before al-Jumu’ah until the imam arrives. After the imam’s arrival, one should no longer offer any salah, save for the prayer of greeting the mosque (tahayyatul masjid) which may be performed quickly during the khutbah unless one comes at the end of the khutbah and would not have the time [i.e., before the actual salah begins] to perform tahayyatul masjid.
Ibn ‘Umar used to perform a long prayer before al-Jumu’ah and then two rak’at after it, and he said that the Prophet used to do so. This is related by Abu Dawud.
Abu Hurairah reports that the Prophet sallallahu alehi wasallam said: “Whoever makes ghusl on the day of Jumu’ah and then goes to the mosque and prays what has been prescribed for him, and remains quiet while the imam delivers the khutbah, and then prays with the imam, he will have forgiven for him what is between that Jumu’ah and the next and an additional three days.” This is related by Muslim.
Jabir reports that a man came to the mosque on Jumu’ah while the Prophet was delivering the khutbah. The Prophet inquired of him: “Did you offer the salah?” The man replied: “No!” He told him: “Pray two rak’at.” This is related by the group. In one narration it states: “If one of you comes to the mosque on the day of Jumu’ah and the imam is delivering the khutbah, he should pray two rak’at and make them quick.” This is related by Ahmad, Muslim, and Abu Dawud. In another narration, it is stated: “If one of you comes to the mosque on the day of Jumu’ah and the imam has already arrived, he should offer two rak’at.” This is related by al-Bukhari and Muslim.