They would have to login to sh.itjust.works.
nope. every instance can post to any other instances groups. my 'home base' instance (moist) has almost no local communities, but the users can interact with all of the lemmyverse
if you look at the group list you'll see they are all remote groups that local users can subscribe and interact with locally
my instance has a 'copy' of the remote content. when a local user interacts with that content, it is sent to the other instance.
its what the 'federation' in the fediverse means. all users can subscribe/upvote/interact with 'remote' instances. the actions of the users are federated between instances.