Praise be to Allah.
It is best for the individual to slaughter his own udhiyah himself. If he cannot do that or does not want to do it, there is nothing wrong with appointing someone else to do it and watching it being slaughtered. If he cannot do that or does not want to do it, then there is nothing wrong with him being absent at the time when it is slaughtered.
Appointing someone else to slaughter the udhiyah is permissible, and there is no difference of opinion among the scholars concerning that. Attending the slaughter is mustahabb (encouraged) but is not obligatory.
Ibn Qudaamah (may Allah have mercy on him) said:
If he slaughters it with his own hand, that is preferable, because the Prophet (blessings and peace of Allah be upon him) sacrificed two horned rams that were white speckled with black. He slaughtered them with his own hand, said Bismillaah and Allahu akbar, and put his foot on their necks; and he slaughtered some of the sacrificial camels that he brought for his Hajj, sixty-three camels, with his own hand.
But if he appoints someone else to do it on his behalf, that is permissible, because the Prophet (blessings and peace of Allah be upon him) appointed someone else to slaughter the remaining sacrificial camels after he had slaughtered sixty-three of them. There is no difference of scholarly opinion concerning this, but it is mustahabb to be present when they are slaughtered.
End quote from al-Mughni (13/389-390)
It says in Fataawa al-Lajnah ad-Daa’imah (10/441): The reward for the udhiyah, if it is voluntary, will be shared by everyone who is included in the intention, even if he is not present. Because the Prophet (blessings and peace of Allah be upon him) said: “Deeds are but by intentions, and each person will have but that which he intended.” End quote.
For more information, please see the answer to question no. 175475.
And Allah knows best.