What happens when put a Finder alias in a Dropbox folder?
The Finder alias will appear as a zero-byte file in Dropbox. Any files linked to it will not be uploaded.

If you would like to have Dropbox sync the contents of the alias, you can instead use what's called a symbolic link. Basically, it's an alias that the operating system (and Dropbox) treat as indistinguishable from the actual folder. This question on SuperUser answers how to create a symbolic link.