I did not see if you had posted this before but is there going to be drinking there? Because if the is I can tell you right now you get that many guys drinking and mix in sex with only one woman and its just bad. Control is going to go out the window so fast it won't be funny. There is no way anyone can tell you what could happen. All anyone can do is give you maybe's. If you love your wife and you want to keep her safe I would really think long and hard about all the maybe's and rape is a big maybe I can see happening there. Talk to her and if she/you think she should still go through with it then go with her if things spiral out of control then you guys have an out.
I just hear a little voice screaming that the whole thing smacks danger.