Simple process:
Create MacroDrag it to your Action BarUse MacroUpdate Macro
Create Macro
The macro editting area can be reached by typing /macro in your chat window.
In the popped-up window, create a New Macro, select an icon to represent it, and in the text window, put this:
/assist SomePlayer
Drag it to your Action Bar
For quick accessability, drag the new macro icon to your Action Bar, and make sure it's hot-keyed for quick execution.
Use Macro
If you're in a party with SomePlayer, as soon as they enter into battle, execute your new macro and begin your attacks. This macro will target the same enemy SomePlayer is targetting, so you'll join in on her fight instead of bringing another into the fray.
Update Macro
Whenever you get into a party, update your macro so it assists the correct person. No use hitting your "Assist SomePlayer" macro when you're partied up with SomeOtherPlayer!
Updating your macro is similar to creating a new one: Type /macro, find your existing macro, update it, and exit the Macros window.
Good luck!