You can have more than one person working on the same branch as you, but that's not recommended. With one person per branch everyone can make their mess without worrying about disturbing someone else's work (like writing code that temporary breaks some part of the application, for example).
… will get back to
.git/objects but for now notice that every directory name is two characters long. Git generates a 40-character checksum (SHA-1) hash for every object and the first two characters of that checksum are used as directory name and the other 38 as file (object) name.