How to set up a multi-developer environment with emphasis on source code security?

One of my clients runs a small software development company (15 employees) with PHP as their sole server side development language. They are having a networked environment with all the computers connected to a central server via LAN. The server also runs a FTP server.

Their hardware setup is as follows:

Individual developer Environment: Windows 7 x64 Professional + XAMPP + Netbeans

Central Development Server Environment: CentOS 7 + Apache + PHP + mySQL

Their current workflow is as follows:

1. When multiple developers are involved in a same project they create a new directory in the central server under a /projects folder using a FTP client (with a common FTP credential that everyone knows).


2. Then they use GoodSync(http://ift.tt/xE7S0C) to sync their files from their local machine to server and vice versa.

Also they are planning to install and use Git in the main server.

Everything works fine but the organization wants to tighten the code security so as to disallow the access of all code of all the projects to all the developers.

So how can they create a simple easy to manage but a secure system for multiple developers?