Visa requirement for group of four, one US citizen (trip sponsor) and three Bangladesh citizen.

I am a US citizen wanting to visit Hongkong with three cousin sisters who are Bangladesh passport holders for seven days as tourists. I will bear the cost of the trip including airlines ticket and hotel stay. What are the visa requirements, how long does it take, can the visas be applied on-line and if there is additional advise for this group trip, please let me know. Thanks