What IS meeting up?
--I'm at home, and Susan is getting off work 15 minutes away. I text her to see if she wants to meet somewhere to get a drink.
What is NOT meeting up?
--Me and Susan are both at work together, and we both decided to have lunch together nearby. (This is NOT meeting up because we started in the same location.)
--I go to a coffee shop, and I happen to see my friend Susan there, and we sit together and chat over coffee. (This is NOT meeting up because the event was not planned.)