How to reset the stats that have gone badly wrong.

I am using this module as part of Drupal Commons.
My stats currently tell me that one group has 1286, another has 203 members.
In fact the first group has about 140 members, and the second group has 17. The whole site has about 140 members.

I don't know how it got to be wrong.
I understand there is a way to reset the stats - but can't find any instructions.

The first group is compulsory, (you get to be a member when you register) and we have lots of spam registrations, so that is probably how that number got so large.
The second group can't be joined until your are approved and log in - and so the spam people can't do that. I guess people could have joined and left the group. Maybe the stats is just counting people joining, not the actual number of members, and not subtracting as people leave.

Your help would be appreciated.

Cheers
Miriam