How do you grow you pre-alpha release subscriber list?

Could you try posting on beta list?

Here is my approach

• Create landing page - I just launched one yesterday https://poorsaps.com (would love some feedback)
• Test different channels
• If there is sufficient interest/traffic setup some split testing
• Communicate with those who opted in and start testing possible product features
• Create a video explaining the benefits/features and add it to the landing page.
• If problem statement/solution explainer are doing well post on some of these
  https://github.com/mmccaff/PlacesToPostYourStartup

Assuming there is a validated use case for your product...

Create an initial user list (few hundreds) manually:

1. Find the channels where your users are (FB Groups? Followers of a twitter/IG account?) People who may be already using similar products.
2. Create a higher incentive to sign up, i.e. if you sign up now, you will get either vanity reward or a better deal on the product, or scarcity
3. Approach the users either via advertising or make a deal with the channel owner
4. Create a referral program if the product is suitable

Once you have the small list, use this to extrapolate for broader advertising e.g. via Facebook Lookalike Audiences.

What you have to keep in mind is that it's better and important to have your first adopters and people who are genuinely interested in both your product and also in supporting your efforts, rather than people who just signed up because there was a freebie, but would not actually use your product.

Good luck!