Two ways to grow:
1) Broaden your audience focus. Facebook is the best example. Facebook jumped from Harvard to ivy-league schools, to all almost all educational institutions to just over everyone in just over two years.
2) Double down on your existing audience. StackOverflow and Kaggle are good examples. Begin with one functional purpose you can help your audience with and gradually find more and more ways to be useful.
For most of us, I’d go with the latter.
Let’s imagine you manage a community for marketers.
Your interviews might reveal that using the right technology remains a big challenge.
You might develop a community dedicated to marketing tech. This might mean a reviews platform, latest news about platforms, comparison guides, and listed rates. You win by becoming the best source of quality information about which tech platforms to use.
If that succeeds you might expand to implementation of tech platforms, migrations between platforms, setting up an approved list of vendors, and how to get the most out of each technology marketers use.
If that succeeds you might move to processes. This might mean having the best list of case studies, tips, building up the best experts to participate and share information.
If that goes well you might move into recruitment etc…etc…
The ultimate goal is to be the destination where your audience can get all the help they could possibly need. But you can’t rush it. You have to ferociously commit to tackling each stage in turn. Target the specific focus at each stage and make sure you put an overwhelming amount of resources into making it work.