The iPod system is designed so that one iTunes library can have several iPods, but one iPod should only be linked to one iTunes library.
If you connect your iPod to a new iTunes library (on your own computer in a different user's area, or a friend's computer), then it'll pop up asking what you want to do: sync with this new library, or cancel. If you cancel, nothing happens. If you (perhaps accidentally) tell it to sync, it'll copy across everything from that library to your iPod. If that new library happens to be blank, then it'll erase everything in your iPod too. The syncing just makes your iPod the same as the library you're syncing from.
However, you can use other programs such as Yamipod to copy more music across, or just put friends' music on a flash drive or something then put it into your iTunes library, then connect your iPod like usual and it'll happily copy across and not do anything to your existing music.