I am trying to share to share two drives on a headless 'server' Windows 7 Pro PC across the network to my 'main' Windows 7 Pro PC and map them as network drives. I've successfully managed to share the drive with 'Everyone' but this isn't the end result I need. At the moment I'm unable to grant read/write permissions on the 'server' drives to my username on 'main'. When I try and add a user on 'server' in the form 'main'/user I hit an error because, I assume, 'server' doesn't who that user is. Is there a way to perhaps change the domain in the permissions from 'server' to 'main'? Any ideas?

If the two PCs are not domain members there is no way for them to see each others accounts. You have to make sure both machines have an account with the same username and password if you want this to be transparent.