Click one of the characters to display the menu. The "Reward" button will show a notification with the number of pending rewards to be given to this person, if any. Click the "Reward" button to display a window with a list of the achievements waiting to be rewarded. Select one of them and, then, select the reward you want to give the person.
Before giving out a reward, it's important to ask questions to the members of your team to get to know the person better.