I don't know exactly the difference between those two? What are the most commons use for those ones?
For example, what are the difference between these sentences?
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友達に集まった does not make any sense. It should be 友達が集まった. But even then the meaning is not the same.
会う : meet
集まる : gather
友達が集まった。 : my friends gathered
友達に会った。 : I met my friends
They aren't related at all.