Create 3 users and set the sharing privileges for the company all to private,
create a group and make all 3 users a member of this group.
Create a new user. Check the user_privileges/sharing_privileges_<user>.php after creation. You will see that the 'GROUP'=>array is filled by the group information you have setup for the other users but this array should be empty.
The fault is in
function getUserModuleSharingObjects
$query="select vtiger_datashare_grp2grp.* from vtiger_datashare_grp2grp inner join vtiger_datashare_module_rel on vtiger_datashare_module_rel.shareid=vtiger_datashare_grp2grp.shareid where vtiger_datashare_module_rel.tabid=?";
...
if (count($groupList) > 0) {
$query .= " and vtiger_datashare_grp2grp.to_groupid in (". generateQuestionMarks($groupList) .")";
If a user has no groups all group information get assigned