I think there is a limit given the format and accessibility of an online forum as compared to networked social media groups on similar topics designed to be shared on peoples streams. I think it is what it is.
I think fostering a respectful and civil forum, focused on constructive support and advice is our strength and priority. IMHO, that is the community, fellowship and Bennington support service we provide very well here.
Growing this forum could be something members “try to get the word out about” on their various social media. For example, I have often referred/referenced this forum to Bennington owners asking questions covered here on my social media. I think if “we“ as members want to grow it, it is up to us individually to get word out about it.
If Bennington is interested in growing it, they could perhaps have their dealer partnership network inform and encourage membership via word dealership mouth or literature. Perhaps during sales discussions or with boat purchases. Frankly, I think we are a pretty supportive group that would probably help close some purchases due to our own generally positive experiences and love of our boats.
For instance, In the past Bennington tied extended warranty years to joining the forum (likely in an attempt to grow this community). Maybe looking into the metrics of that effort could guide future corporate ideas and efforts, cost benefit of such things, etc…
I am sure there are a number of marketing style things that could be done.