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It's probably best to talk to everyone you can find.
However, you can only enter someone else's Black City over local wireless - no online/Internet. The Pokemon Black player has to more or less be in the same room as you.
I just wanted to add that you have to make sure the person whose Black City you're accessing isn't in the entralink also or you won't be able to go there either. Only one of you can be in the entralink at a time.