An Opt-In List is a group of people who have agreed to receive email from other members of the group. Because they have to give their email address to the list manager, they have "opted" to get "in' the list.
There are many different types of opt-in lists on many different topics. Some are open for anyone to join. Some are exclusive. Some are free and some cost money. Some are moderated (a person decides what incoming email to send out to the people in the list) and some are unmoderated (anything that gets sent to the list address, gets forwarded to the rest of the group). Some accept ads and some do not.
Some lists generate a LOT of email every day, others spit out a little bit. (because of that, you may want to use one of your "junk email accounts" for joining opt-in lists.
The 6-Figure Tips discussion is an opt-in list -- it's actually a moderated discussion list.
You can find lists of opt-in lists at http://www.onelist.com/ and http://www.topica.com/
You'll also hear about opt-in lists from the email you'll get back when you do various promotions.
If you go to topica.com or onelist.com you'll see instructions about joining the list. Similarly, when you get email about a list, you'll get instructions about joining.
Typically, joining a list is as simple as sending an email to something like listname-subscribe@domain.com - Under Construction
Once you subscribe to a list, you'll get an email back with confirmation that you have joined and instructions about how to participate, manage your account, etc.
NOTE: Once you get on the list, follow the instructions to receive the list's email in 'digest' format. A "digest" is a collection of the emails that the list is sending out, combined into one email. Sometimes the digest is sent out based on time (e.g. once a day or once a week). Sometimes the digest is sent based on volume (e.g. every 50 emails).
The way to promote in an opt-in list is PARTICIPATE. Get into the conversation in the list and, when appropriate, point people to your 6-Figure site... or to the FREE Internet marketing training they get when they join.
Luckily, many opt-in lists are practically nothing but ads, so participating with those is merely emailing one of the Sample Ads to the list's address.
When you join, read the guidelines. Some lists let you advertise every day. Some let you advertise every week. FOLLOW the rules!
One easy way to mail to the lists you subscribe to is this:
You can do this same thing in different ways, e.g. by making email 'stationery' in your email software, by simply saving the outgoing email with all your Opt-In addresses and re-sending it... I'll leave it to you to figure out the most efficient way to do this.