http://www.hktrampers.com
They organise stuff mainly in the cooler months. If you want the summer stuff I would look in their links sections. There are a few sites with members who hike year-round.
My husband and I will be visiting Hong Kong for a few days and would like to join a group for a hike on Sunday, March 4, 2012. Does anyone know if there is a hiking group that hikes on this date and would be willing to add the two of us?