AP fr US to HK, to 中国广东，再往返 HK广东 多次，在香港要什么证件

Hi friends,

I have AP now, I need to go to China GuangDong and HK for business trip.

I will stay in China GuangDong for 2 weeks. During this time, I need to go to HK several times since our company has both offices in HK and Guangdong.

What kind of document I need to get to HK for several times. ( by the way, my round trip ticket will be from US to HK)

Thanks you so much.