So many good suggestions here. I would like to share some of my experience that confirms many of the above. I ran several support communities that you only had to sign up for if you wanted to contribute yet growth was not equal between them. One was very slow and organic and mainly word of mouth. At that company community was very isolated and the product and our customer facing teams did not promote the existence of this amazing community at all.
At the other company community was integral part of the product and it was part of customer onboarding, training and the support cycle. We had crazy sign up rates because everyone was told to do so. Read 500 or more a week. But not many stuck around. Mainly only the product admins did who had come to learn and gain customer support. Others joined because they were told to do so and needed to check the box.
What this long winded example is trying to showcase is, yes track where the entry points are to your community?. What efforts are being extended to drive sign up, i.e.: in product referral, training efforts, marketing etc?
To answer @HAWK’s comment, yes GA is amazing at tracking your referral sources and entry and exit points. There is no magic behind it. Just copy the GA code snippet into the website HTML code and let it do it’s thing. Referral source will give you broad understanding of search engine vs social media vs. email vs. organic vs. bookmarked traffic, entry points will give you literally the sites they come from and where they go to most frequently. (Exit page is the opposite)
I highly recommend picking a few of the quiet people and set up one on one’s with them. I would target the ones with repeat login but no or barely any engagements. Why do thy keep coming back. It might be a case of “lurkers” and you just need to activate them.