Find people who have the same taste of music as you do who play instruments.
It matters what genre of music you want to play. If you want to play rock you probably want a guitar, bass, drums, keyboard, and vocals.
I'm in a chamber music group that consists of two acoustic guitars, a saxophone, violin, and flute. We play songs like Scarborough Fair (Simon and Garfunkel).
If you want to start a jazz band, lots of horns is good. Saxophones, trumpets, trombones, French horns, clarinets, flutes, piccolos, tubas. Then you also need a rhythm section( drums, piano, bass).
Find some sheet music (either in stores or online) and listen to the song. If all of you like it, you can sight read the music.
You can add harmonies or bass parts. You may have to transpose to a different key for other instruments.
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