Does your friend already have a Plex account? You can’t sign in without an account. I believe if you scroll down on that page, you will see the option to “sign up”, which lets them create a new Plex account.
He does have a Plex account
Not according to that screen. Also keep in mind that they need to accept the invite using the same email address that matches the Plex account. i.e. they get the email to me@me.com but then try signing into a you@you.com Plex account, it won’t work.
You can PM me their email address and I can check what is going on.
How do I pm you?
Click on my username then select “message”.
Can i get an invite? 
