"meet" can be used anytime, but "meet up" means you had plans and is more informal - therefore, you have probably met before. Also, you can't add a pronoun in "meet up" without an extra word and you can use "meetup" as a noun. Let's meet (up) outside the restaurant. Did you meet (up with) him after work?

That is correct. Though, meet can be used for either depending on the region you're in. For example, here in California people use it interchangeably. "Let's go meet them." with the "them" being strangers on the other side of the room and they say "Let's go meet them." with the "them" being their friends that they are meeting up with for lunch. But in general, yes. Meet refers to somebody that you don't know yet and meet up refers to getting together with someone that you already know.