cp file1 file2
where file1 is the existing file and file2 is the destination.
cp /users/guest/msmith/mystuff/example1 smithexample (copies the file 'example1' from the 'mystuff' subdirectory in msmith's account and saves it as 'smithexample')
cp another /users/guest/msmith/mystuff/example2 (copies the file 'another' to the subdirectory 'mystuff' in msmith's account and saves it as 'example2')
cp another yetanother (makes a duplicate copy of 'another' and saves it as 'yetanother', in the same subdirectory)