The only way that I have done it is to put a spade through the root clump and then hopefully prise a piece off without breaking the spade it might not be the most succesful time to do it this time of year especially after all this dry weather but I have planted transplanted clumps next to a stream and under trees in a dry spot and both have survived and multiplied . You can get those decorative living bamboo ornaments that just appear to be just shoved in water and they grow well so you could always try the old favourite and drop a green shoot in a milk bottle full of water so that the bottom half is submerged and then you can watch the roots forming but be carefull I read recently that some types of Bamboo can grow 1metre per day in the wild !!!
I believ you can take a sort of cutting by cutting a section of the growing (green) stem, making sure you include one of the joints. Bury that horizontally in some warm, moist compost and it shoudl gro a new shoot perpendicular to the cuttting at the joint. I have never tried this but it works apparently - not sure if it applies to all bamboos either.