Hello, to help with the process of ranking challenges, we have badges now for the people who are willing to officially judge them, just to help you know who to ping. Don't ping anyone who doesn't have one of those badges.

There are two badges, Tier 1 Ranker and Tier 2 Ranker.
Tier 1 means that user can rank Tier 1 challenges
Tier 2 means that user can rank Tier 1 and Tier 2 challenges.
We never explicitly restricted people from being able to apply for being a ranker, but that piece of lore got lost in the rework, it seems.
So let me explicitly state again that anyone can apply to be a ranker, the only requirements are that you've demonstrated being knowledgeable enough and sensible enough for the job.
Send a DM to someone with a ranker badge if you're interested.
Don't let not being a ranker stop you from offering constructive criticism, just make it clear that it's not official and be nice.
Last thing, let me state something else again for the record:
When a ranker gives you a grade, that is 100% official unless they made an error. You can consider yourself as owning the grade you were given.
The only reason it's just admins that can give a badge for a 4/5 or 5/5 grade is because of limitations with the forum software. If that wasn't the case we'd gladly give those powers to the moderators, sentinelles and even the rankers. And the giving of the badge is 100% a formality, the admins generally don't even check the ranking result to see if it's correct.