Recently I modified a group registration from the back end, a user had registered without logging in to the site. To allow the user to modify their registration I went in the back end and attached her username to the registration. When I saved the change the number of group members changed from 2 to 1. When I clicked on the group members both members were listed but only one was counting against the number of registrants. Has anyone else seen this?
This is an issue that has already been addressed and posted as fixed. Update to the very latest package and you will find this does not happen any longer.
Once you update, you will need to manually fix any records that were modified to show "1 Member". To fix them, go to your database in the table jos_dtregister_user (of course the jos_ prefix may be different for your site). The last field for each record will be "memtot". This shows the number of members for any registration record. Modify this so the counts and calculations will work correctly.
2.7.13a is the current package. You can see it in the announcements category of this forum with details of that update. We also post announcement of any "smaller" updates on Facebook, Twitter and Google+ so be sure to follow at least one of those.