Sending and receiving lives from your friends is a Facebook only feature. If you are on a mobile device, you can connect to Facebook as soon as you open your game or through your game settings. If you are playing through your computer, you'll automatically have the ability to send and receive lives from your friends.
When your game is launched, if you have not sent lives yet for the day, Panda Pop will prompt you with a list of your friends that have a Panda Pop account, allowing you to send them lives.
From here, you can choose "select all", or only select individual friends.
After you have made your selection, simply press send!
If your game didn't prompt you to send lives to your friends, another method is to open up a level and see your friends' score at the bottom of your screen. Just below their picture, you will see a heart with a +1 icon. Select the heart and you'll be able to send them a life (but remember, only once a day!).