In the name of Allah the Most Gracious the Most Merciful
Arriving late and entering the Jumu’ah salah while the Imaam is in Tashahhud
Answered by Shaykh Abdul-Aziz bin Abdillah bin Baaz may Allah have mercy on him
Question: If I entered the masjid on the day of Jumu’ah and I caught the tashahhud for the prayer do I continue and pray 2 units or do I pray Dhuhr with 4 units? Also if another man was praying behind me and he did not catch the tashahhud does he have to pray the Jumu’ah Salah or Dhuhr?
Answer: It is upon you all to pray Dhuhr because the Jumu’ah salah was not caught. If it was unprecedented and he missed the two units he should intend to pray Dhuhr with 4 units. The one who is following him should do the same. That is because Jumu’ah Salah is only attained if one catches a unit (meaning at least before one raises from rukoo’). It was narrated from Abu Hurairah that the Messenger of Allah (peace be upon him) said:
“Whoever catches up with a Rak’ah of the prayer, then he has caught up with the prayer.” [Collected by an-Nisa’i]
حكم من أدرك التشهد من صلاة الجمعة
الشيخ بن باز رحمه اللهإذا دخلت المسجد يوم الجمعة وأدركت التشهد فقط من صلاة الجمعة، فهل أصلي ركعتين أم أصليها ظهراً أربع ركعات، وإذا صلى خلفي رجل آخر لم يدرك التشهد هل يصلي الجمعة أم الظهر؟
عليكم أن تصلوا ظهرا؛ لأن الجمعة ما تدرك إلا بركعة فإذا جاء المسبوق وقد فاتت الركعتان يصلي ظهراً ينويها ظهراً أربعا. والذي يقتدي به كذلك ظهراً، لأن الجمعة إنما تدرك بركعة واحدة؛ يقول النبي – صلى الله عليه وسلم -: (من أدرك ركعة من الصلاة فقد أدرك الصلاة).
Translated by: Abu Awzaa’ee AbdusSalaam
الفقير الى الله
Trying to Follow the Salaf 
Shaykh Salih al Fawzan adds also that there is no second Jammat for salatu jumah so he must pray Dhur Salah if he miss jumah. This was very good reminder. From the last question on this video of his http://www.youtube.com/watch?v=QSlvP3vNkcc