Within the next few days, we plan to release a major update to the standard Stats page that appears in your community's admin section. You can read all about the details of the update here, but I'd like to point out one specific chart for you. We've added an "Active Users" chart that's designed to shows you the users who've had the most activity in your community over the designated time period (either a week, month or year). That chart will be a really good indicator for identifying new all stars or champions. :)
When you guys talk about "growth" in the community, what types of growth are you looking for? Google Analytics is a really good tool for monitoring overall page views. The new stats page will also give you a breakdown of new topics, as well as new comments/ replies, so both of those will be a good indicator of whether or not activity is increasing or decreasing. Lastly, there's a count on your community's home page of both the total topics posted & how many people have joined the community. You could always keep an eye on those numbers too.
Let me know if there's other metrics that you're looking for and I can brainstorm with the team on how to get them!