I'm being prompted to upgrade when I am viewing someone else's Dropbox files?

Thanks for your post, ND3000. 

That’s because any files they upload within the shared folder are also added to your own Dropbox account causing you to go over-quota (if the total amount of files surpasses the 2GB). You can also read more info here.

If you want them to share files with you, which won’t take up account space, they can make a shared link and send it over instead.

Let me know if that helps.