Text field “validation”

I need a text field where you can only enter a specific text from a list of words. I came across this site (http://contactform7.com/2015/03/28/custom-validation/) and since I know very little PHP, I can’t figure it on my own.

[text* your-name]

When you enter your name (p.e.: “John Doe”) it should check if that name is in that list of names – if the name is not in the list, then you can’t fill out that form field. I guess this can be achieved by editing the validation code in the functions.php.

I need this for a Website, where only allowed visitors (about 30 names total) shall be able to access the page by entering their full names. It doesn’t matter if it’s not secure – it’s just to make the page more personal.