How can some of my L$ be transferred to my friend

[Storm Clarence]

Pull up their profile when you are in-world, and click on the 'pay' button on the bottom.  Fill in the amount you would like to send.  

[Storm Clarence]

Pull up their profile when you are in-world, and click on the 'pay' button on the bottom.  Fill in the amount you would like to send.  

[Ansariel Hiller]

As addition to the answer of Storm: If they are in sight, you can also right click on them and choose "Pay" from the context menu.