Do you look at product stickiness? Or user group members' overall engagement, such as in references, community, or other marketing?
Also looking at customer education stats—are your user group members the most educated users of your product, for example.
And beyond just possible revenue in the room, I'd also recommend looking at the length of deal cycles—as I'd estimate that your user group members could be contributing to faster deal cycles on renewals or upsell/expansions.