1. If it's possible to share dreams, does that mean that if I see a friend in a dream, that he will be having the same dream? If not, how do you differentiate between people who are real dreamers and just figments of your imagination (which I believe is what all DC's are)

There is a tutorial on that in the beyond dreaming forums. DCs are created by your mind, and if you ignore them, they are gone. Otherwise, if you try to completely ignore someone in a dream and it doesn't work, then it's a dreamer.

2. Do both you and the other person have to be lucid to share a dream?

Hypothetically, no. The non-lucid person may not remember it though.

3. Wouldn't it hypothetically be possible to share a dream with someone I've never seen, and then accurately describe that person physically with no previous knowledge?

Yes. It probably wont be accurate, as people's appearances may be unusual or distorted in a dream.