I'm really surprised that Dropbox didn't already offer this feature, but the file-sharing service now allows comments on its shared files, just like Google Drive. This'll make it a lot easier to add context or ask questions about files you're sharing in a group.
To start a comment conversation, you tag someone's username or email address with the @ sign. Even if they don't have Dropbox, they will be able to weigh in.
Right now, the feature is only available on the web, but mobile compatibility is on its way. Competitors like Google Drive already have similar commenting services, so Dropbox is playing catchup here, not innovating -- but this is a welcome, overdue feature for a popular service.