Q: Using the new Photos app with Dropbox

Kinda clumsy but you could import the photos from dropbox as referenced files, but you'd need to do that on each Mac regularly.

You cannot put your Photos Library into a Dropbox folder. Syncing will corrupt it.

If you want to use Photos and sync photos across devices, use iCloud Photo Library. That is the main new feature of Photos.

You're welcome.

This explains a bit about the iCloud Photo Library:

      iCloud Photo Library beta FAQ

And this explains, what will sync between Macs:  Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support