It is very common for our guests to have guests. This is something we can all expect to run into in the future. If for some reason, a 2nd guest does stay with a 1st guest and it didn't get placed into the reservation, then you can request additional money from your guest after the fact--from the Resolution Center on Air BnB--for the second guest, if you require that payment.
Many hosts give a price for a room that covers two people, having only three or more guests be paid for separately. So, many of our guests automatically expect (especially if they don't know Air BnB well,) that they can bring their own person with and have them stay overnight--why not? Because they paid for a room, they can put anyone they want in that room, goes the thinking.
Be aware that if you charge a separate price for a second guest, some people will think they can bring that person in without knowing they need to pay.