Problems with Group ownership on sharepoint

I have created my network users, and by default they belong to group Staff, and I haven't changed that. My problem; when user A creates a folder on the sharpoint and user B tries to access this, user B has no right on that folder other than read access. How can this be, when both A and B belongs to group Staff? Shouldn't user B have read/write access on the folder? running server macos X 10.2.1 regards