You'll need an Apple device to set up Family Sharing (for the 5 users part), but if you don't need that you can set it up in a browser.
I haven't gone through the process yet, but it additionally looks like I will need to borrow a Mac or Windows machine to start up iCloud Email. Once it is enabled I should be able to use IMAP or their web client.
That said, if you don't have anyone in your family with an Apple product, this iCloud+ thing may not be right for you.