Lando - clearly he was just there for surveillance. Perhaps he figured that stealing Han would be too risky, as he could be caught and be either imprisoned or executed.
3PO and R2 being given away (as well as Luke's lightsaber) - perhaps Luke had a plan to get them back through negotiations. Otherwise, droids are somewhat expendable. Imagine offering to give away your car in exchange for a chance to get your best friend freed from captivity. You may not want to, but ultimately it's just a car, and can be replaced. A person cannot be.
Leia and Chewie - Leia needed a clever way to gain access to the palace. That's where Chewie comes into the picture. Jabba had a price on his head.
Luke - since all else failed, he was the last chance.