There is a mail icon located under the amount of stars you have. That will only be visible when you have a new mail, either a request or an answer.
If a Facebook friend have sent a request for a life, the mail icon is visible. If a Facebook friend have responded to your request and given you a life, the mail icon is visible.
You can only receive 5 lives max at the same time. So even if you have requested lives from 10 different friends and they all gave lives, only 5 is usable and shown in the mail.
The lives that you have received will stay in your mail until you use them, You can use them whenever you want, except when you have full lives (5/5, 6/6).
When you are going to ask for lives (from the life menu) you get redirected to a list of all your Facebook friends. There you can check a box next to your friends name and then click on "Send/OK" to send the request to all the checked friends. You can also search for friends in that view. You receive one life per friend and you send one life per request.