Now that you mention it, he does confirm in ARR that he was once of the Circle. Even the way he interprets his study for the Sons is in line with the Circle's thinking - cataloging the past to protect the future. My only guess is that he wasn't high enough in the chain to bear the marks back when the marks were being handed out. The others showed up in 1562, whereas we don't meet Rammbroes until 1572. The others have been on important missions to interfere with a coming Calamity, whereas Rammbroes is just hanging out at the Gold Bazaar as a pitstop on his journey across Eorzea cataloging how important knowledge diffused across the realm and then vanished from it. He's even pretty detached from the threats Eorzea faces as a whole, preferring to stick to his scholarship (though, he's found his calling in the Allag, apparently). If my interpretations of the marks are correct, it's the most logical assumption I can think of, but I've no proof.
Another complete shot in the dark? Our point of view as Eorzean adventurers has been on Minfilia and Louisoix's groups and we just don't get to see all of our allies. Minfilia's made a lot of connections since Louisoix's death - a lot of networking has been done and many of his allies are now ours. G'raha is of the Students of Baldesion, who had access to a lot of Sharlayan resources that Eorzeans couldn't get ahold of. Perhaps the Students had links to the original Circle, perhaps some were members before Louisoix took off (perhaps some were assigned by him to places we didn't see). G'raha strikes me as one of the youngest members, anyway, and he seems to love labels. He's got the keepers of the shelves "archon" tat, he's got the "scions" tat, he's of the Students, hanging out with the Sons, and puts a new label (NOAH) on the combination effort to explore the Crystal Tower. Strikes me as an over-achieving, exciteable kid that grows up a bit as a result of what he learns in the Tower.
I wonder what, if any, tattoos Krile will have, and Galuf, for that matter, assuming he's not dead.