Step 1: Click on this icon. The following box will pop up:

Step 2: Find the person you wish to add by typing in all or part of their name or number (turquoise #1 below).

Step 3: A list of people who matched what you typed will appear (turquoise #2 below).

Step 4: Click on the person you want to add (turquoise #3 above).

Step 5: Click Done to save your changes (turquoise #4 above).