All perfect praise be to Allah, The Lord of the Worlds, and may His blessings and peace be upon our Prophet Muhammad and upon all of his family and companions.

Al-Imam Ar-Ramli (May Allah have mercy on his soul) said (What means): "It is permitted for a praying person to perform the prayer of salutation during Friday's sermon but he is obliged to make it short since Sulaik Ghatafani came on Friday when the Messenger of Allah (PBUH) was delivering the sermon. He (Sulaik) sat down. He (the Holy Prophet) said to him: "0 Sulaik I stand and observe two rak'ahs and make them short, and then said: "When any one of you comes on Friday, while the Imam delivers the sermon, he should observe two rak'ahs and should make them short." [Muslim].

That is in case he performed the two supererogatory Friday's Rakas at home, but if he didn't, then he should perform them shortly without performing any other Rakas since this will avail him for performing the prayer of salutation. Moreover, as for the person who enters the Mosque at the end of Friday's sermon, if he believes that he will miss the opening Takbeer of the obligatory prayer with the Imam then he shouldn't perform that prayer (Salutation) rather he should stand and never sit down in order to perform the obligatory prayer.

Ibn Al-Ref`ah said in this regards: If this person who came at the end of Friday's sermon performed the prayer of salutation, then it is recommended for the Imam to make the sermon a bit longer until the former finishes his prayer." [Nihayat Al-Muhtaj 2/321]. And Allah knows best.