The fact that the Imaam has commenced the Khutbah does not foreclose the late comer to the masjid to observe a two-unit prayer called Tahiyyatul-masjid - the prayer of greeting the masjid.
The Khutbah is no barrier to observing it.
The Messenger of Allaah - sallallaahu alahyi wa sallam - saw a man enter the masjid while he was delivering the Khutbah on a day of Jumu'ah, the man sat without observing two rak'ah, he - sallallaahu alahyi wa sallam - commanded to observe it and he added:
'Whenever any of you comes on the day of Jumu'ah while the Imaam is delivering the Khutbah, let him observe two rak'ah and he should be swift in them.' Recorded by al-Bukhaaare and Muslim.
It is therefore a profound ignorance that the Imaam should prevent the late comers from observing the tahiyyatul-masjid.
As a matter of fact the Sahaba used to talk when the Imaam first mounted the mimbar, they would only stop when the muadhin stopped the adhaan and the Imaam began to talk.
See partly: Al-Qawl Al-Mubeen, pp. 369-371.
In the past edition: