It is the mask tools. Depending on your background, the choice of mask tools will vary. If you have a very plain single color background, that is of high enough contrast to the image of the head, you could use either the Lasso, magic wand, or magnetic lasso. Then, invert your mask, and creat an object of the masked head. However, as these often times yeild less then desirable results for me, I almost always use a combination of the magic wand and the brash mask.
If the bacground is very cluttered, or not very contrasting, it is usually easier to start by masking the face, and cut it out of the background.
1.) Zoom into the face to fill most of your work area
2.) select magic wand, and click the flesh portion of the face
3.) select Brush Mask and click the "+" on the toolbar at top to "add" to your mask and select a suitable sized brush. Often times I find it easier to have "0" tollerance for the "feathering" option, but a little feather (about 20%) can allow for a little less precision.
4.) Paint the lines out to the "outline" of the head - just up to the background. You can fiddle with the "+"/"-" keys and paint the mask to get exactly the outline you want. If you aren't to concerned how "clipped out" the image looks, a quick once around with the Brush Mask, or even the elipse tool, will be sufficient.
5.) CTRL+UpArrow or Object-> Create -> Object: Copy Selection. This will make your mask appear to have disappeared (which it has) only now the mask is trnsformed into a new object that copied the contents (the shape of your mask) from whatever was "behind" the mask shape.
6.) "Copy" (CTRL+C) this object.
7.) Open the Invitation project and "Paste" (CTRL+V). The heac will now appear in your new layout. Just click on it and move into location. Rotate it, skew it, etc, to make it match your new body.
This should give you enough info to get started meesing with the appropriate tools.