The answer to this one is short and sweet – only those who opt-in to your list.  If you just arbitrarily add people to your mailing list, you will be accused of spamming (and they’ll be right!).

Its called an opt-in list for a reason.  If they don’t enter their name and email address in your web form, or email you and tell you they want to get on your mailing list, they shouldn’t be on there.

If, for example, somebody sends you an email to ask a question, you CANNOT add that person to your mailing list.

If you are on someone else’s mailing list, you cannot add them to your mailing list.  The popular line of thinking, especially among people who are new to Internet Marketing, is that if someone sends you their newsletter (because you opted in to get it), then you can add them to your newsletter.

It doesn’t work that way.  They did not opt-in to receive your newsletter, so you cannot send it to them. DO NOT DO THIS!

The ONLY people that you can add to your mailing list are people that have specifically signed up to receive it.  Period.  Anything else, and you’re taking a huge risk.