Can't believe it hasn't been mentioned yet, but Nextcloud (or Owncloud):
It's basically self-hosted Dropbox, with clients for all mayor Desktop and Mobile OSes. I set it up for a little team project. Just one account, and a shared folder where people with a password could upload. I think we will move to individual accounts at some point.
But it supports much more. It has a calendar similar to Google Calendar and I've switched to it. It also has plugins for image galleries, contacts, LibreOffice in the browser, collaborative editing like EtherPad, and so on. I was very sceptical, but it is really well done.