Quote Originally Posted by wereotter View Post
you're not far enough in the story yet to join a grand company. This is actually part of the main scenario after your character completes a major upcoming feat that has all three grand companies petition you to join them. It can't be missed, just keep progressing the main scenario and you'll get to it soon!
Yea you literally can't miss it. It's impossible to complete the main story without choosing a grand company. There will come a point in the story in which you must choose who to join and you cannot progress until you do.