AV and ETTL are your problem. Stop using them for this sort of thing. Put away the Speedlight* and get a radio trigger.
Set the camera on manual mode, 1/160 shutter, ISO200, f/8.
Set the B800 to whatever power level will properly expose the subjects. Start at 1/4 which should be pretty close unless the light is very far from the subjects**. Trigger the B800 with the radio trigger/receiver.
* If you want to get fancy, you can use the Speedlight as a hair or kicker light. Set it to manual mode, it'll need maybe 1/8 power. Put the radio receiver on the Speedlight and set the B800 to optical slave mode so you don't need to buy a second receiver***.
** If you still need more power, you can open up the aperture to f/5.6 and maybe bump up the ISO. But the B800 should be plenty even at 1/4 or 1/2 power, unless it's really far away from the subjects, so if it's at full power and still underexposing, you're doing something else wrong. If 1/4 power is too bright, turn ISO down to 100 then start dropping the flash power until you get proper exposure.
*** If there are other people around who are also using flashes, optical slave mode is not a good solution; you'll need a second receiver for the B800.
I just did a photo booth this weekend with a single Elinchrom D-lite-it 2 for my light source; camera settings were (I think) 1/160, f/5.6, ISO200. The D-lite is only a 200W/s unit, and it was firing through a strip box with two diffusion panels at about 1/4 power. The light was about 8 feet from the subjects. Your B800 is 320W/s so you should have plenty of power for a simple photobooth unless you're setting up the light far, far away from the subjects.