Catch me up is slang and an informal way of saying "talk to me later" or "let me know". It is grammatically incorrect and moreover isn't used so often in English. For example, if you asked someone a question and they didn't know the answer however still wanted to know, they could say, "Sorry, I'm not sure but catch me up later if you find out." "Catch me up" isn't as common as "hit me up" is, however that term is used mostly between teenagers and could come off as quite awkward if older people were to use it (but by all means, if you want to say "hit me up" then go ahead!)

"Catch me up" means to give me information so that I understand what has been going on. One example: If you walk up to a group of friends who are talking about something interesting, you could say, "Catch me up on what Somayeh and Ahmed are fighting about now." Or, if they are watching a football game, and you arrive when the game is half over, you might say, "Can you catch me up on the game?" Hope this helps. :-)