Marketing Growth - How to build a massive community?

Typically you can't force these kind of things and generally the way to do it is one by one, focusing on your current users, helping them succeed and creating a culture they would want to be associated with.

What are you willing to do for your people?

In my experience, the fastest way to truly grow a community is by tapping into your closest network first and gaining that initial traction.

For my product, ruttl.com, I initially managed to gain some traction through my linkedin connections. Once the ball got rolling, I started working on executing a carefully crafted social media marketing strategy. This involved making new content and using email marketing campaigns.

If you can also identify the right target audience for your product, you should start approaching these people on the most popular platform amongst this audience. In my case, it was Dribbble.com and Behance.com, since I am solving an issue for the design community.

Hope this helps you.
Cheers!

Do you know your Ideal Customer Profile? Have you niched down to a specific vertical yet? If you're just starting out going laser focus on your audience will allow you to know who your prospect is, connect with them, get feedback, and incorporate feedback that helps all of the people in that vertical.

If you focus on a niche you can also find online communities where these people already are at and join their community and help out. If you have a good product and a couple start using it then the word will spread. People in the same vertical often talk to other people in that vertical.

@Kacisse Make sure the new unknown SaaS solves a problem, reach out to users, gather their input and feedback, use this and them, as a cornerstone for the community.