I am using Secure Invites v1.2.2. Under Signup settings I have specified my Signup page URL to start with https. But invitation emails contain a link starting with http which is not appropriate in my case.
As a side-effect, because I require HTTPS on login/register/admin pages, the http page is redirected to the proper https page. This would work OK, except for the other issue I noted: the @ symbol in the invite URL is not URL encoded, so WordPress (being careful about such things) ends up stripping it out during the redirect.
I suspect this would break invitations for anyone using HTTPS. I hope this report helps improve the Secure Invites plugin -- I'd really like to be able to use it for production purposes! Thanks for your hard work.
http://wordpress.org/extend/plugins/wordpress-mu-secure-invites/