Enplane is the opposite of deplane. In other words, it is to get on an airplane in preparation for flight.
Who uses a word like 'enplane'? The same people who use the word 'deplane'. Airport statistics often contain records of enplaned passengers and deplaned passengers in a given period. These numbers can be different if there are an especial number of one-way passengers. If you add enplaned and deplaned passengers you can get an average number of passengers through the airport, though stopover passengers are counted double or not at all.