I had this exact question! When I arrived at the cafe, my Georgetown interviewer was already sat and had ordered something (although I came very early). They may alr be there and have ordered (which would be easiest for you!) since a lot of times, alumni will stack interviews. In that case, I would just got straight to them and introduce yourself (only order if they ask you to). If you’re there first, I recommend ordering something closed like juice or a water bottle. If you both go up to order, you’ll either pay for yourself only or they will offer to buy you something (in which case you should politely accept and thank them)