The formatting !community@instance should let a user click through but still be logged in on their own instance, so that you can still read posts, vote and comment. If that doesn't work, you can try entering that same thing into your instance search bar, or in your browser enter https://your instance/c/community@theirinstance
It should all get a lot smoother as this platform is developed!
I haven't tried linking to a specific post on another instance, so I'm not sure of that formatting hmm