Good question. There is a subtle difference. The addition of "up" in the expression makes the request sound a bit softer. In British English, we don't say "wait up", but instead "hold on" or "wait a sec". These two expressions are softer than simply saying "wait!", which might come across as a bit too direct.

great question...but I never learn it as long as I learn English but as long as I know, when u say "wait up", it means u include object there. the most common "wait ME up", "do u wanna wait her up?" when u try to ask someone in front of u to wait u up, u say "hey,wait" "wait" means directly at that time, instantly u ask that person to stop any activities and wait u up. I hope it helps.

when you're walking on the street, you see someone ahead of you, you say , "hey mikey, wait up", now that means I have to stop walking and wait till you catch up so that you can speak to me" when you say wait, you are not necessarily telling me to stop doing whatever I am doing at that particular moment. when you meet with someone, then the purpose is often formal such as a business or something important. When you meet up with someone, you are just gathering together to do some social stuff, like shopping together. You can (I'd say) always use meet instead of meet up, but you dont want to use meet up when you are going to a formal meeting with people you had never met before or people that are not close to you.