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If you want to use Time Machine on one Mac & share that disk it to to others you need to use OS X server. I doubt you want to run that on your 'day to day' iMac.

https://www.apple.com/osx/server/

A Time Capsule is designed to share one backup disk across multiple Macs. Time Machine with a disk attached to Mac is intended for that Mac only without the server.

File syncing is not really built into Mac OS (at least not in a simple to configure 'GUI way'), you will need to investigate applications that can be set to sync folders. Chronosync is one application. http://econtechnologies.com/

You will need to enable file sharing on the Mac that you want to copy onto & then connect to it via the network.

How to connect with File Sharing on your Mac - Apple Support

Chronosync can use 'network shares'.

Syncing files isn't trivial - you have to consider if you aways want the latest version of files. If dates are out of sync you can overwrite newer documents, backup before you sync is my advice. If you try to sync everything it can get messy (preferences & cache files etc should be excluded or synced via iCloud) - be selective if you create your own system.

Another option is to use a service like Dropbox. That way you can allow files to sync via the internet, provided the iMac is on it will download the latest changes & stuff them into it's Time Machine backup. iCloud drive is another possibility. It depends on the file sizes & your mobile network connection if you plan to sync via the internet.

Do you already have a network setup or are you hoping to setup a router/ switch/ wifi etc?

Having owned a notebook and a desktop ever since Apple introduced its first real notebook, figuring out a way to keep everything in sync has been my search for nerdvana. Thanks to Apple and DropBox I’ve reached that zen state. (DropBox isn’t the only solution but it is the one that works best for me.)

For a long time I used to use networking and a folder syncing program to keep my documents in sync. That was fine except I had to remember to do it, it was sometimes a pain in the neck, and when I skipped a couple days I sometimes found that I had edited the same document on both computers which meant I had to manually resolve those edits. Unfortunately that system was even more imperfect when it came to keeping my contacts and calendars in sync.Apple’s cloud technology came to my rescue for keeping contacts, photos, calendars, todo lists and passwords in sync. If you don’t already have an iCloud account, make one. It is free and includes a minimal amount of iCloud drive space. But the important thing is what I’ve described above.

Now for documents. In the early days I found Apple’s iCloud too slow and unreliable for document syncing and DropBox was fast and free for a reasonable large amount of space (back in the day). Early adopters also had the chance to add extra free space and I did. Eventually I purchased a subscription and because it is so popular iOS developers have also adopted it as one of the defacto cloud services their programs support. There are other choices including Box, Google, Microsoft’s cloud etc. Take a look around at the various services. The thing to look for is a service that includes software that automatically syncs the folders you choose to the cloud and then from the cloud syncs them to your other devices. So that’s your documents sorted.

Now for backups. I don’t consider DropBox a backup service even though it does hold about 80% of my documents. In my opinion TimeMachine over the net is a good idea that in real practice doesn’t work out as well as it should in theory. The problem is that even if backing up works flawlessly all the time if/when it comes to restoring the hard drive, it has to be done over the network. WiFi is too slow to restore more than a few files and even though a fast Ethernet connection should work well I’ve seen TimeMachines stall all too often with an Ethernet connection. For this reason my preference is the low tech fast external drive.

dwb, Thanks for the reply post, I'm in the same boat and the last line of your post is the same conclusion I came to and am using myself.

You don't need server to host a Time Machine drive. Just Share it out and tell each Mac to use it. TM should mount the drive, back up, then unmount the drive.

I haven't run into the issues dwb notes, but there are many threads discussing problems. I believe you can connect he drive directly to restore, but I won't be able to test that for a few days. That wouldn't be possible with a Time Capsule, though.

See the late, great Pondini's Time Machine FAQ: http://pondini.org/TM/22.html