There is no specific GIS software for doing this: most will handle the RGB image and the Lidar data.
Basically, NDVI is (NIR - RED)/(NIR + RED). Most of the time, aerial Lidar gives you the NIR value (to be checked in metadata) and the first band of your RGB image gives you the RED value. Just make sure that your data are calibrated to reflectance (or, if not possible, normalised in the same range of value -> this will give you a good discrimination, but no absolute threshold can then be used).
As a remark, Lidar does not always provide the intensity information. Sometimes only the distance information is available.