Put part of a photo into another photo?

I want to isolate a girl from a photo and put her into a group photo that she missed out on being in. How should I go about this? I am using Mac OS 10.4 and Photoshop CS2. I would really appreciate some advice on the subject.
Thank You!!!

Copy the picture over.

eliminate the background or do whatever you need to do to blend the pic in with the group shot.

A good start will be to read up on Layer Masks, the Extract Filter, and the Pen Tool in the User Guide or the Help files in Photoshop.