The above advice isn’t perfect - in general, use Search first.
Until the first person searches for a community on your instance, your instance doesn’t know about it and the URL won’t work. Searching (usually using the [email protected] form) should always work.
Equally, your instance doesn’t start pulling data on a community (messages etc.) until someone subscribes to it, and then when subscribed it will sometimes take a a few hours to get it all (it should immediately start getting new posts and comments).
See the other message I have written - you should always recommend search. The URL only works if someone has already searched for that community. If you’re on a big instance, that’s almost always true, but on a smaller instance it often isn’t.