Hi there - at one level, there isn't a defined time when people started making music together - hunter-gatherer peoples the world around appear to make music together, though clearly their music works in different ways to that of a symphony orchestra or a rock band. So the point when a group of people with instruments got together to make music is almost certainly prehistoric. At another level, people before the Middle Ages did talk about music, and the part of the FAQ that deals with music in antiquity has some great responses from /u/erus, /u/racecar_ray, and /u/verbee about music in ancient times.
We don't know. We have depictions of musicians from antiquity, and every reason to think people were making music in this way long before those were created.
For example, here's a band in ancient Egypt, and people were probably also singing/praying/both in groups . We can find depictions of the same instruments being played together.