One e-mail headline that I have used with great success is

"Hey [firstname], personal JV invite (Please Do Not Share)"

For some reason, that e-mail gets people to open every time.
And then my sincerity in the e-mail gets them to reply

These are some great tips in this thread. Proving that you are
a customer or a fan of their work is an amazing way to grab
their attention as well!

Matt