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I don't think you need platinum...he isn't a very high level, but you probably wont get him on your first try. oh, and make sure you have enough space for him. otherwise you'll never get him.
Probably because your level was too low. Groudon has a join rate of -10%. So if you want to have a chance to get it to join you need to defeat it with a pokemon that is lv60 or higher. But of course , you can always equip yourself with the Friend Bow.