Because of this, users who are members of a common group, but who have other primary groups, may create folders with varying ownership inside the folder of their common group. Members of this group may expect to have access to everything inside of the common group's folder, but that won't be the case.
To resolve the issue, log in as an administrator, select the affected folder, and Get Info. In the Ownership & Permissions section of the Info window, change the ownership to the desired group.