See the "!" button under everyone's avatar?
Just click on it. Then I think they need to agree to the challenge and/or move first.
I wouldn't really recommend challenging other users though, since it doesn't give you anything (no money, no xp, no item, etc.), except maybe some bragging rights.
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On semi-hiatus all the time during school year.
I'll reply to post whenever I can.
PM me if you need a quick answer.