Should prayer for the deceased at the grave be offered together?

Shaykh Ibn Baaz (may Allaah have mercy on him) issued a fatwa
stating that it is permissible to offer du’aa’ together, so one of the
people attending the funeral may say du’aa’ and the people say “Ameen” to
his du’aa’, or each person may offer du’aa’ individually. 

He said: 

The Sunnah that is proven from the Messenger (peace and
blessings of Allaah be upon him) indicates that it is prescribed to say
du’aa’ for the deceased after the burial. When the Prophet (peace and
blessings of Allaah be upon him) had finished burying someone, he would
stand over the grave and say: “Pray for forgiveness for your brother, and
ask that he be made steadfast, for right now he is being questioned.”
Narrated by Abu Dawood, 3221; classed as saheeh by al-Albaani in Saheeh
Abi Dawood. 

There is nothing wrong
with one person saying du’aa’ and the listeners saying “Ameen”, or with each
person saying du’aa’ by himself for the deceased. 

Majallat al-Buhooth al-Islamiyyah,
68/53. 

This is with regard to
its being permissible. However what is better and closer to the Sunnah is
for each person to make du’aa’ individually. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said
concerning du’aa’ offered together: 

This is not part of the
Sunnah of the Messenger (peace and blessings of Allaah be upon him) or
the way of the Rightly-Guided Caliphs (may Allaah be pleased with them),
rather the Messenger (peace and blessings of Allaah be upon him) used
to tell them to pray for forgiveness for the deceased and ask for him to be
made steadfast, individually and not together.” 

Fataawa al-Janaa’iz, p. 228. 

And Allaah knows best.