If I start a newsletter for a website and plan to email all members signed up... do I have to give them a way to "opt out" or can I just place somewhere on the site "by signing up you agree to receive updates/announcements" ?
You will certainly run in to problems if you don't give them a way to get out of it, just because they put their e-mail in there, it may be their friends and not theirs. And how would the real owner stop receiving them then? A lot of people put fake e-mails, and besides you would run into other conflicts in the future. Just put a message at the bottom of each newsletter stating, "click here to no longer receive the newsletter", if they like your service, they will still want to receive them.