There is the memberlist (Members in the top menu) - though that contains many duplicate and spoof accounts. But you can tell how many active users there are. Forums typically attract a tiny proportion of project users. Moon Zoo is no exception. Pity really as the forums are where the interesting / unusual stuff is posted, and where the science team look for help on mini projects. It's a fairly fluid population - "regulars" come, stay for a while and then and go... Forum life!