Or, select everyone as a group and immediately disband that group. Then you're free to pic individual units.
I imagine if you ctrl H, you can tell them to heal, then issue orders to who's left, but I would then want to disband the group that was healing and order them to heal as individuals so I would be prompted to give orders to each as they returned to 100% rather than being prompted to give an order to the whole group only after the last one healed...