janiskr,
If their systems are compatible with yours, you can use
which will package up a package you already have installed. Be warned it also packages your current settings files, not the defualt ones normall provided with the package.
You can also instruct emerge to buid packages (not to install them). They will go into /usr/portage/packages by defualt. Burn this to CD for your friends. There are several ways to get make.conf right, from passing parameters on the emerge command line, to setting environment varables to editing your own make.conf.
You could also install their hard drive in your PC and do an install on their drive, with make.conf set up for your friends PC.
I have used most methods.