TankAssist creates a macro key, called Assist, that makes it easy to set whom you are assisting, even if that character is using special characters in their name. To change whom you are assisting, simply target that character, hold shift, and press the key to which you've assigned your Assist macro.
Note: Due to restrictions placed by Blizzard, it is not possible to change a macro key while in combat. Changing your tank on the Assist key will be reflected once you are out of combat.
Additional Note: Please remember that if your keybind to your Assist key is on your number pad on the right of your keyboard, pressing shift while pressing this key will trigger the non-number-locked function of that key rather than the number.