Yeah I looked into this as well. I forgot the actual courtcase but a military man had a medical card and the military (air force) was kicking him out for popping on a drug test. The case went up to supreme court and they favored the military...so sad I know so your answer is no you can not join the military even with a MMJ card. Don't let this stop you from going into the service though, do what your goals and ambitions are and weed is just a extra little bonus.
Don't ever let the ganja prevent you from doing what you really want to do.