Welcome to Orchestra Party
People have been putting instruments together in various combinations for as long as there have been instruments, thousands and thousands of years. But it wasn't until about the last 400 years that musicians started forming into combinations that turned into the modern orchestra. In the old days, when musicians got together to play, they used whatever instruments were around. If there were three lute players, a harp, and two flutes, then that's what they used. By the 1500s, the time known as the Renaissance, the word "consort" was used to mean a group of instrumentalists, and sometimes singers too, making music together or "in concert".
Early Renaissance composers usually didn't say what instrument they were writing a part for. They meant for the parts to be played by whatever was around. But around 1600 in Italy, the composer Claudio Monteverdi liked things just so. He knew just what instruments he wanted to accompany his opera Orfeo (1607), and he said exactly what instruments should play: fifteen viols of different sizes; two violins; four flutes, two large and two medium; two oboes, two cornetts (small wooden trumpets), four trumpets, five trombones, a harp, two harpsichords, and three small organs.
What we do at Orchestra Party ...!
You can see that Monteverdi's "Renaissance orchestra" was already starting to look like what we think of as an orchestra: instruments organized into sections; lots of bowed strings; lots of variety. In the next century (up to about 1700, J.S. Bach's time) the orchestra developed still further. The violin family, violin, viola, cello, and bass, replaced the viols, and this new kind of string section became even more central to the Baroque orchestra than the viols had been in the Renaissance. Musical leadership in the Baroque orchestra came from the keyboard instruments, with the harpsichordist, or sometimes the organist, acting as leader. When J.S. Bach worked with an orchestra, he sat at the organ or harpsichord and gave cues from his bench.