What is the best way to grow the database for an international investment company without spending a fortune on advertising?

Firstly - it is not always about spending a fortune on marketing and advertising. It is more about laws of physics than math. If you can put enough force into things that all increase the energy of that force - meaning, doing less isolated efforts of marketing but more integrated approach.
Look at your target audience, where do they usually go for advice on international investments - Google ad-words is a good start but again, that is where active seekers would be found and you should have planned and implemented a good SEO approach. For passive prospects (not looking actively for international investments)- you would still need to generate interest through branding campaigns. This is again where, if you plan it well, perform research on the right channels for adverting to your prospects, some are free online, some are worth the spend. Also, look at your partners, and co-marketing campaigns in this time are a real asset as well.
In a nut shell, there is not a cookie cutter solution and not one size fits all. If you spend more time planning, you will spend less money and less time will be wasted on execution of your marketing and advertising campaigns.
I would be happy to have a call if you want to discuss this in further details and share some of our experience with such campaigns.

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?

Hi!

I would say use the power of Content Marketing and Email communication. It's cheap and it reaches people you want to target. This is an ongoing process though and it takes some time. So depending on the urgency I would try to go for this.

However, be sure to have a clear strategy and plan before you start communicating, e.g. Buying Personas, Content, Funnel offers...