Buy and bolt in three sets of bus seats with seatbelts. They are often cheap and available on CL. Either that or put people at some elevated level of risk. A seatbelt wrapped over a wooden kitchen chair will be insufficient if called to duty. IMO
A lot of it is down to the anchorage. You can have an ordinary sofa (so long as the sofa is firmly fixed to the floor to carry its own weight) and -- if the anchorages are solid -- the seat belts will work fine. IMO - if it's a situation that puts you in a position that your passengers in a bus* need shoulder straps, it doesn't matter; you're all already going to be dead. Especially, if a passenger's seat if not near the windshield, instrument panel, etc., lap belts -- worn correctly -- are good enough.
(* A bus has significantly different characteristics to a smaller vehicle. Generally, if you hit something hard going very fast, that "something" is usually a car or small truck and the greater mass of the bus will give you a lot of protection. If you hit something hard when you're going fast and what you hit is BIG and HARD, nothing in the front 4-6 feet of the bus will survive. But belts are great to provide quite good protections in side-swipes, in-traffic bumps, and slow intersection accidents; plus every driver should be belted to keep the driver in the seat during "difficult conditions" (don't forget, many accidents are multiple, you hit something that deflects you into hitting something else -- if the driver is thrown from the seat by the first hit, from then on, you're all just passengers with no way to influence things.)