How does it get different if I just say "met" not "met up"?
"Met up" is a phrasal verb of "met", and usually means that you join or get together with someone, either because you planned to or because it happened unexpectedly. "Met" or "meet" usually means to be introduced to someone for the first time. "Meet" can also mean to get together with someone, however, it is usually planned and not an unexpected meeting. "Met up" and "meet" in some cases may mean the same, but "met up" is always a little more casual. Ex: I met up with my friends after class for lunch. Ex: I met the new manager yesterday. He seems nice. Ex: He met with the client to discuss a business deal.