I just started a curated newsletter about personal finance for millennials. Each issue includes 10 curated articles from various sources about investing, budgeting, paying off loans, and etc. I do not have any subscribers yet.
The most reliable way to drive sign-ups is to run ads to a landing page with a strong content offer. The positioning of the offer - title, content, core message - being the key to success.
There are many other potential tactics, as described below, but they are extremely time intensive and often have sporadic and unpredictable returns.
Paying for the traffic and optimizing over time will be your most reliable and predictable path to progress.