If you use Jomsocial and you want to subscribe your users to your newsletter during the registration on your site you'll need to activate and configure this plugin.
Make sure that the User - CMC Registration plugin is turned on! Otherwise this plugin won't function properly & your users won't be subscribed to the newsletter when they click the activation link in the confirmation email.
Read the chapter on "User - CMC Registration plugin"! The fields that are required in mailchimp have to be present in the newsletter form, unless you've configured a field mapping. The standard field mapping for email, first name and last name looks like this:
EMAIL=jsemail FNAME=jsname:0 LNAME=jsname:1