This is the problem I am having too.
When I network my two laptops any external drives attached to them appear on the linked computer. But in neither direction can I get the boot drive (or any area of that drive like my documents for instance) to appear on the other machine. So, since the installation of High Sierra 10.13.3 I cannot synchronise my computers. I had thought setting permissions for the boot drive to include me by user name (rather than the generic 'admin') might help. But apparently that's impossible because it's a very bad idea.
I can, however, easily link with my wife's computer (as a registered user) that's still running 10.12.6. The update to Sierra hasn't affected that network connection. It's just between my own two machines.
I know the high tech numpties would prefer I rent space in the cloud and synchronise my computers that way. But they are clearly never out of range of high speed wifi. Some of us spend time in less populated areas with a 1 meg internet connection. SO I would really appreciate it if someone can tell me how to retrieve the functionality I had.