Word of mouth, references, advertisements? I've been using Reddit heavily to connect with the community my platform is based in. But I don't know how else to grow at scale.
Good question. I'm actually just helping a startup with this.
Suggestions (some obvious, some less):
1. Forums (in your relevant service).
2. FB groups.
3. Partnerships with HR people in that field.
4. Post Job opening.
6. Contact them directly, and give them an incentive to post/share that they've joined your platform (which will bring some of their friends).
There's a lot more - depends on the field.
I've successfully helped over 350 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david