How can I find all files that do NOT contain a text string?

Your find should work if you change -v -l (files that have any line not matching) to -L (files with no lines matching), but you could also use grep's recursive (-r) option:

grep -rL shared.php .

find . -type f | xargs grep -H -c 'shared.php' | grep 0$ | cut -d':' -f1    

OR

find . -type f -exec grep -H -c 'shared.php' {} \; | grep 0$ | cut -d':' -f1

Here we are calculating number of matching lines(using -c) in a file if the count is 0 then its the required file, so we cut the first column i.e. filename from the output.

I know that grep -L * will do this, but how can I use the find command in combination with grep to exclude files is what I really want to know

You could use find and grep like this:

find . -type f ! -exec grep -q 'PATTERN' {} \; -print

Here -print is executed only if the previous expression: ! -exec ... {} evaluates as true.
So for each file found, grep -q 'PATTERN' is executed, if the result is false then the entire expression ! -exec grep -q 'PATTERN' evaluates as true and the file name is printed.