Welcome all! Pernisciouse is on the right track. I have done an investigation on this because I wanted to know the ranking system better a while back, and these titles are granted by trophy points. I am not sure about the exact numbers, but
A jr member is someone that has less that 100 trophy points. A member is someone with over(I think) 250 trophy points. Well known is 500, and sr.member is over 1000 trophy points. I know there are more ranks than that, but there is currently noone with enough points to reach that rank. The ranks also uses to be covenant based, but to make it more understandable, easy access, and to accommodate site upgrades and a further rendition to the trophy system, it was changed to what wa have now, and to prioritize being in a regiment, instead of a rank. Here are the old ranks
Recruit - 0
Grunt - 50
Drone - 100
Jackal - 200
Brute - 500
Hunter - 1000
Elite - 2000
Prophet - 5000
I may be totally wrong with the numbers and the titles changed but the amount of trophy points did not...