There's no place that lists the available number of levels you can grant. If RAF is set up correctly, you should see the ability to summon or grant levels listed when you right-click the player portrait in game. The recruiter's account should also show a confirmation under the RAF section of their BattleNet account page.

To grant a level, you must be within 100 yards of the character you are granting levels to.

The character you are granting levels to must be lower level than the character you are granting levels from.

http://us.battle.net/support/en/arti...friend-rewards