The Sendy integration we provide for Joomla is for newsletter signup only. There is a signup module which you can let users add their email address to on the front-end and a user plugin that will add users when they register to your site. Once the user is added to Sendy, all administration and management of users and the newsletters themselves is handled within Sendy.
Our integration does not let you pull information from Joomla into Sendy when you send a newsletter, which is what it sounds like you're looking for. The email address and name of the user, when they register, will be available in Sendy, but extra fields or other information would have to be set separately.
I hope that helps explain, but should you need anything else, please let us know.
Thanks,
Alex