There are several ways to leverage an existing email list to get more email addresses and/or customers.

First, I'd make sure that your emails to these folks are optimized for sharing. Include a call-to-action at the bottom to share the content/product/offer with their friends by posting to social media or forwarding your email.

A referral program is another great way to incentivize people to share. It's tough to give more specific suggestions without knowing what type of product/service you have and who your audience is, but try to think about how you can provide additional value to your customers if they sign up X new customers or share your content with X friends.

If you're using content marketing as a way of attracting leads, you can run a test where - instead of filling out a form to download your content - they have to share the content on social to unlock the download (called "share-gating").

If you do set up a referral program, using Custom Audiences on Facebook is a great way to target ads to your existing customer base - use this and email marketing as your channels for growing awareness of your referral program.

Hope this helps - happy to hop on a call if you'd like to discuss further!

You want to think about the value you can offer. You never want to address what they can do for you. It's the other way around. If you are adding value to your audience they'll be more inclined to evangelize for you. What you are talking about is really a lead gen campaign. I would suggest you create a lead magnet product and drive traffic to a landing page while trading that product for their email address. It's not that simple, really, but that's the base playbook. You can either purchase or earn that traffic, but most of the time I see people pay for the front-end funnel and make their money off the back-end.

Send your existing customers to a page to enter a contest.

When they enter the contest, immediately send them to a page offering to increase the prize if they share and comment.

I think Sarah's response we the best you've received.
To add to her thoughts and your feedback;

I would consider using service Pay With A Tweet, assuming you leverage your email list good enough to where they open your email and click on your links, I would recommend directing them to a Pay With A Tweet promotion, where they can continue to your site, ‘purchase’ an ebook or other offering by sharing with their social networks (they can pay with various platforms not just tweet)

Also, I would create landing pages or the main pages to be optimized with A/B variations to test what drives your bottom line more.

Along with that, try modifying your current email’s content. Instead of covering various topics try narrowing down an email to just one item, this will reduce your click through but your sharing should increase – the idea is that highly interested subjects will have similarly engaged networks with whom they want to share something valuable with. You don’t want to engage every single subscriber in every email, but with several emails pushed with different topics you should increase sharing and even click through.

One simple great step to this is creating a targeted survey through email asking what they would like to see more of.

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Everyone has offered great advise. I see two things mentioned by David & Huebert, needing elevation. Providing value to the 800 you already have. Are they opening and clicking?
You can add people to the list, but if they aren't responding what good is it. If you're providing value all your other efforts will be much more fruitful.

First question I have is how engaged is your email list? What are their open rates, their click through rates, etc? What is their product? What have they offered them in the past? How many times have they bought? Do they continue to buy? What is their greatest need? Do they align with your target customer?

If they are engaged and opening emails then it becomes a matter of rediscovering what is relevant in their world of needs. Run a survey and get a great feel for what's important to them. Write content they want to share. As someone else mentioned use "share-gating". Those are all great ideas.

An important part here is what you're using for your email marketing. I would definitely recommend using a service where you can do behavior driven marketing, such as Infusionsoft or ActiveCampaign (depending on your budget).

Behavior driven marketing will trigger marketing actions based on engagement, such as visiting a page, watching a video, opening an email, etc. This is so important. So let's say you have a group of people who you send an email to with a button that says "check out this video and tell your friends", then all those people who click on the link you can tag those people and trigger a new campaign that will further target them.

Basically you can get granular on who really is engaged and nudge the others who are not.

I also would definitely focus on increasing more revenue first by getting my existing 800 customers to buy more times more often. I would focus on creating targeted valuable content in the form of webinars, videos and content or offers pertaining to their needs.

Then I would go into lead generation by developing out funnels that will build a list wherein the marketing pays for itself. Then I would optimize the funnel till I got it to the place where for every $1 I spend I would make $2 back.

The challenge I've seen with most businesses is they get caught up looking at marketing in terms of what it's going to cost instead of what it's going to make them.

The first and most important part of scaling your customer list is getting your new lead gen traffic to break even with some form of an offer. Getting emails in a landing page is great, but if you're not liquidating your traffic and making the cost break even on the front end then you're going to go broke really fast. The key here is to create what's called a Self Liquidating Offer or SLO. If you knew that for every $1 you spent you made $2 back, how many $1 would you spend?

The first area to begin looking at is what your sales process and your sales funnel will look like. Both on the front end and on the back end. The first step is to get them to enter in their information with an incentive of great reward (video, pdf, free ebook, etc). Realize most people will give their fake email on the first submit. So the next page is really important. This is where you want to take them either A) into a sales video with small "dip your toe in the water" offer that's around $7 to $25 or B) provide them with another valuable content but then have a form on the page where they enter their real information. After they have completed steps 1 and 2 then you take them into your real offer, the big buy.

Now on the subject of using retargeting, I would take all your unsubscribe emails and create custom audiences to drive them back into your funnels. For your existing customers I would create a custom audience in facebook ads with content or incentives to share with their friends or straight out target their friends. Again using behavior driven marketing and automation you can trigger campaigns to further target engaged people.

So there's a lot you can do here.