Firstly, someone can only sign themselves up to your mailing list if you have chosen to allow members of the public to join the mailing list, when you set up the mailing list for the first time. (You can change this setting in 'Mailing Lists' by clicking on the list whose settings you want to change, then clicking on the grey 'Settings' button - not the Settings button at the top of the page).
When someone signs themselves up to join your mailing list, they will receive a confirmation email from interests.me, checking that they really do want to subscribe. This is to prevent people signing others up to hear from you.
If they confirm that they do want to join your mailing list, they will usually receive the latest email newsletter which has been sent to the mailing list they are joining. This only happens if there has been an email newsletter sent in the last 30 days. This means that they can see what they have signed up for.