The best way to grow the database would be to have a clearly defined target audience. Next you want to put your compelling message in front of the audience. Provide a clear way for the target to reach you and to continue to receive your valuable content on a consistent basis.

Since you mention investors, you'd have to look at levels of sophistication, risk, and net worth. Someone who will invest $60K is different than someone who has an disposable $1M. So you should further segment that audience. What value do you add that highlights the investment opportunity and makes it compelling? You need to get personal with these investors. And are you trying to reach the investor or their representative? Can you host events on the property or provide more exclusivity if someone subscribes to your updates?

You'll have to become an educator if you're talking about social housing, alternative energy etc. What's the risk from municipalities, environmental groups, how patient does an investor need to be? These are things that could be answered for those that are willing to join your database.

Finally, what credibility can you demonstrate? Do you have referrals and successes? Are you writing in the
publications your audience reads? Are you a guest on the shows they watch or listen to for investment advice?
If not you, who is recognized as the most authoritative in this sector? How do you compare?