I can answer that! You know of that "Basic Pump" you have in your inventory that's not equipment? As well as those little squares at the bottom of the window that light up depending on which num-key you click?
First, use Q to open a special window. Then select one of your choice methods to inflate the NPCs. Then you click C while next to an NPC to use it on them. The squares show which item you can use when you select them.