I've seen questions here and here and also tried googling but it's still kinda confusing to me. Is there any difference between the meanings of "a meeting" and "a meetup", and when can/can't I use one or another?
A meetup is less formal, which tends to be of less importance.
A meeting is the appropriate word and tends to be of greater importance.
You can see a meetup as an informal reunion with not important results at the end of it, and a reunion where is more about socializing.
You can see a meeting as a formal reunion where the participants desire an important change at the end of it.
If you can, always use the word meeting.
"Meeting" is more appropriate than "meetup" in formal situations. You normally wouldn't say, for example, "I arranged a meetup with a potential client." You could, though, use "meetup" for getting together with friends and acquaintances, or for groups of people who have arranged to meet informally.
"Meeting" also usually suggests that there is a specific purpose for getting together with the other person or people (for example, to talk business or discuss something important), whereas the people coming together for a "meetup" could simply be getting together to chat or hang out.
We have a very important meeting at 2 pm at the Big Lebowski conf room. The highlight of the meeting is the meet-up between all the collaborators of the project to initiate the project. And then at 8 pm, we will have a dinner meet-up at the Giant Tree hotel to continue in cordial and informal manner what we would intend to accomplish at the Big Lebowski meeting.