Who are the other people?
I do not think that the other people are anyone in specific. They are simply stand-ins for all the people who have existed in the history of the world.
I do think it is important, though, that the one person who is identified individually is a preacher. I think that is because Hardy is trying to tell us that the ideas of Christianity sure have not done much good in the world.
But, in general, the other skeletons are no one in particular. All of humanity is at fault for the mistakes that the speaker (and God) is talking about. So there is no reason to single out any particular people to speak in the poem.
The other three voices heard in the poem are those of God, another skeleton, and Parson Thirdly. Their traits are revealed through the qualities of their individual speeches. God is scornful, ironic, and amused, but nevertheless is not without compassion. The other skeleton, unnamed, is disillusioned. Parson Thirdly is regretful about his life of piety, and wishes he had enjoyed himself when he was alive rather than leading a life of preaching and sacrifice for forty years. This makes an unequivocal statement for the enjoyment of life and the ability to make friends and stop fighting with one another.