This seems to be the crux of an organization. Since Bob has X ability shouldnt he have rank? No, no he should not.
Let me explain;
As stated previously, ranks are an organizational and communications tool. Throwing rank at folks because of a niche ability (although kind and well intentioned) is not helping.
If someone has a useful or specialty skill allow them to operate in that role, it doesn't mean they need a higher rank.
For example; Bill joins the militia. He’s wet behind the ears on many things. However, he worked at a local state/national park and he is great at land navigation and knows a bit about foraging. Bill can teach land nav effectively. Have him teach it, he doesn't need rank for that.
Fred joins and he ran a range for the LEOs. He can be a safety on range days and not need rank.
I hope you’re picking up what I’m putting down.
If all that's true, when and how do I rank people?
In the beginning, when its you and a few friends, agree on who’s in charge. If its a hand full of you the highest rank will be a corporal or sergeant (this is covered in growing your ranks).
Later on it’ll depend on who is active, shows up, is proficient, and isn't a power hungry ahole.
Proficiency are up next.