Ref. No. 1022/41-193
In the name of Allah the most Gracious the most Merciful
Then answer to your question is as follows:
(1) More than one person can participate in the big animal for isale sawab to (to reward) the deceased. But no more than one person can participate in a small animal. And if you aim to reward multiple people, then you could do so by making a voluntary sacrifice on your behalf. (2) If you want to just send reward to more than one deceased person in small animal, you can do. (3) Regardless of whether sacrifice is obligatory on a living person or not, he can depute someone else (make wakeel) to make sacrifice on his behalf. The sacrifice made by the wakeel from him will be valid.
One can also make sacrifices to reward the living people. This can be done by sacrificing a goat on one's behalf and passing on its reward to the living people.
And Allah knows best
Darul Uloom Waqf Deoband