Sheikh ibn Baaz- may Allah have mercy on him- says it is permissible to do supplication as a group for the deceased at a grave, so one of the mourners supplicates and the rest of them would be saying Ameen to the supplication, or each person supplicates individually. He -may Allah have mercy on him- also says: The established sunnah of the Prophet has indicated the legitimacy of supplication for the deceased after burial: When the Prophet finished the burial of the dead he stood over it and said, “Ask forgiveness for your brother, then ask that he may be strengthened, for he is now being questioned” narrated by Abu Dawoud (3221) and was authenticated by Al Albani in “Sahih Abu Dawud”.

So, there is no harm in having some of the attenders supplicating and the listeners follow with the word Ameen, or everyone supplicates individually for the deceased. [Islamic Journal of research].

The above mentioned is assessed from the permissibility context, However, what is better and closer to the sunnah practice, is to have every individual supplicates by himself. Sheikh ibn Uthaymeen- may Allah have mercy on him- says regarding group supplication:-
“This is not from the sunnah of the Prophet , nor is it from the sunnah of the Rightly guided Caliphs (RA) rather the Prophet advised them to ask forgiveness for the deceased and ask him for steadfastness, each by himself, not in a group.”