I'm having an issue inviting other players via email.
When I hit the email icon the following appears"
"Email not available. You need to register your email account on this device before sending an invitation through email."

But I am signed in using my email and I don't see a setting that applies.

any help would be appreciated.

Did you try sending an email (using regular email) to contact@subterfuge-game.com?
Also, for posts like these, please put them in Bug Reports!