Hi. I came across this thread when searching how other people have handeled verification of the 2nd guest. We've only been hosting on Airbnb for about a month and a half and both times we had two guests we had problems.
The first time, the 2nd guest (unregistered with Airbnb) used our toiletries, was very messy, and spilled hair dye all over the bathroom runing a towel, rub, pillow case, and the dye is permanently stained into places on the bathroom wall which will need to be painted over.
The second time, just this past weekend, was a couple and they got in a physical fight (one guy attacked the other after finding out he cheated on him) and I had to go over to the room to remind them that we were home and they were guests in our home, then the agressor just ran out of the apartment. Things were calm for the remaining two days of the reservation. The 2nd guest left his ID in the room so of course I Googled him. I found two public arrest records.
In an effort to avoid these types of situations in the future, I made a few modifications to my account. 1) I added to the rules that I require the LinkedIn profile of the primary guest and the second guest, if they have no recommendations and positive reviews. I figure any doofus can make a FB profile but LinkedIn is more of an intentional act and conveys a level of professionalism; 2) I raised the price for for the second guest.
Other recommendations in this thread to ask for the IDs of both guests and cross-reference their names with other accounts, especially if you are in the house with them, is a great idea and I will be doing that also. Thanks for the ideas.