Looks like Sheldon, Raj and Wolowitz were clearly too busy for their group dinner date with Penny. They have important things to do... like play Jenga!
Sheldon: I'll have a diet Coke. Penny: Can you please order a cocktail? I need to practice mixing drinks. Sheldon: Fine... I'll have a virgin Cuba Libre. Penny: That's... rum and Coke without the rum. Sheldon: Yes, and would you make it diet?