My good friend Kirk McElhearn has an article up at Macworld on syncing multiple iPods to one Mac:
When you're alone with your iPod and your Mac, it's easy to manage your library and sync your music, videos, audiobooks and data. But with the holidays over, many family members who didn't have iPods beforehand may now find themselves the proud owners of one of Apple's music players. And when two or more people share a Mac, it gets a bit confusing. There are several ways you can sync multiple iPods to one Mac, but the easiest method is to share a music folder. Here's how you can share your music with another user on your Mac.