Nice to meet you here.
The term Chapter is commonly used to designate a branch of a larger organisation or club, you are correct that "national group" may also have been acceptable.
Here is an excerpt from the FAQ section of the forum, it is a brief explanation of the name tags user groups. I hope this answers your questions.
Q: What do the Name Tags / User Groups mean? A: Members who use our forum are divided into groups which help determine their levels of access. Some members can freely post while others can move the posts of others or may even ban users. Some common tags are:
Asgardians are members who have verified their accounts and are logged (into the system. These members cannot moderate forums, delete posts, or create announcements.
Chapter Advocates / Chapter Mods are staff who organize national, division, or city chapters of Asgardia. These people can moderate forums, edit certain posts, and modify announcements.
Global Admins / Global Mods are staff who can manage discussions in any forum on the board. These people can moderate forums, edit certain posts, and modify announcements across the board.
Interest Admins / Interest Mods are staff members who manage discussions in the interest forums. These people can moderate forums, edit certain posts, and modify announcements.
Translators are members who volunteer their services to help translate documents and material for Asgardia. They have access to special staff forums to help with their work.