To sell this one you may tell a quick disaster date story about asking someone out, telling the crowd that you will need their help. To do this reenact your scene by building a couch out of three folding chairs covered by a Large sheet or blanket (it must cover all the chairs well).
After building the couch in front of them, pick a girl and sit her on the end chair. Then pull up three guys studs to come give you their best line and move. Demonstrate for them that it all counts here - talk, walk, distance from the girl - that when told to do so they will enter the room one at a time, walk over the the girl, deliver their best lines, and sit right next to her with their arm around her.
After explaining and demonstrating, briefly send them out of the room. While they are out of the room, you pull out and put away the middle chair, stand on end of the sheet and keep the girl sitting on the other.
When the guys deliver their line and begin to sit down, both you and the girl release pressure on the sheet and they will fall right through. Note: the person who take them out should have three lines ready for the guys in case they can't think of one. Make sure you build them up afterward, and you may want a pillow on the ground under the sheet to break their fall.