I don't quite understand the auto-reply with invitation.
My understanding is that a person visits your site, they like what they see so they decide they want to join. The click on the request to join button and fill out the fields in the form for the criteria to join. You get an email with the form info and review it. You decide to accept the request and email the user with the link to the registration form so they can join your site. You want to use tokens so that the person can not forward the email to anyone else and thus have others joining outside of the request feature. The token can only be used once. You can try and set this up using Dolphin's form processor but I would probably do it using a third party script that I use for form processing on sites that is quick and easy to install and configure and can allow attachments in case you request such things as photos to be included in the request. See http://www.tectite.com/, they list pay things but the form processor is free to download and use, they are trying to sell other services.