Add devise_invitable
Created by: Kytanos
add devise_invitable gem to invite new users by sending an email.
email content : "Hello
Someone has invited you to http://localhost:3000/, you can accept it through the link below.
http://localhost:3000/users/invitation/accept?invitation_token=2WsEc5qg_yaxq4kx1hyx
If you don't want to accept the invitation, please ignore this email. Your account won't be created until you access the link above and set your password. "