USA passport holder to get a chinese visa for PRC

from phil .we are planning to go to hongkong this nov 15 to 18.2013 .we are 4 in the group 3 phil.passport holder and 1 american passport holder and we are planning to visit mainland from hongkong .my question is can we apply in hongkong or it is better to apply in the phil?and our companion who has an american pasport is she allowed to enter mainland?and where can she apply if she is allowed?thank you very much in advance if you answer my questions.