Trying to answer this but can't see anywhere that I can do this.

I'm trying to set up a membership site. Only users who have been invited can register so no payment gateway is required. As there may be a large number of users I do not want to be registering them manually. I want them to be able to register themselves.
I'm using Login With Ajax for logging in.
I'm trying to achieve this using invitation codes. I've got Require Invite Codes ticked but I don't know where this field is then supposed to show or how to set this up.

Are you using a plugin for the invitations? If so, which one?

Please advise,

I am using the membership plugin