Do we have to send an invite for the Arcade or can we just send a link out and have people click the link, sign in and get started?

Unfortunately, it is not possible to simply provide a link for unregistered individuals to use. A user account needs to be created before a user is able to sign-in to an Arcade. That can be done through Direct Registration, by adding the users and sending them an email invitation to create their password to complete the registration. The other option would be to connect a Single Sign-On account to the Arcade.