When any person create a new file in the project, he will 
add it to the Source Control System in the correspoding to 
folder. This process is called "Checkin".

Most of the source control systems provide a windows 
explorer like user interface. You can checkin files in 
different ways:

1. Drag and drop files from windows explorer to appropriate 
folder in source control explorer.
2. Go to appropriate folder in source control system, right 
click on the folder name and select 'Add Files'. This will 
launch a file browser which will allow you to select files.
3. Integrate with Visual Studio - most source control 
systems are integrated with Visual Studio when you install 
their client software. This is the easiest way to work with 
source control systems. When you right click on any file in 
the Solution Explorer in Visual Studio, it will give you 
the option to checkin or checkout files. 

After you add (checkin) a file to source control , the file 
is "controlled" by source control system. If anybody want 
to change the file (including the person who created the 
file), he has to "checkout" the file from Source control. 

When you checkin a file to source control, it will make the 
file in your computer 'Read only' so that you cannot edit 
it. This is to remind you that the file is controlled by 
source control system. 

You can add new files to source control, but you cannot 
checkin an existing file unless you have checked out that 
file.