If someone has a group or clic they hang out with, but it's on the bigger side of more than 5 people (so they're never all together at the same time), and they make a time to get everyone together to just hang out, it would be a hanga.
From a get-together: this applies to friends you haven't seen in awhile.
From a party: This is an event at a place, not a hang out.
From a hang out: A hangout has less than 5 people.
In a hanga you see all the people you invited regularly in a structured area (ex. at school, at work).
Lastly, if you are a part of more than one group, then a hanga includes your biggest or closest friends from each group.
Note: A true hanga means all your friends happen to be free on that day. These are the key people in your "group."
"Hell ya! Who's gonna be there?"
"Well, let's see... Rachel, Meghan, Robert, Alex, Gina, and Jazzy. I'm also inviting my friends Mac, Brad, and Elina."
(all friends come to the hanga, except maybe one or two, and they chat and such...)