User blog comment:FallenShadow/Fallen's Reviews: Chapter 341-343/@comment-84.13.41.227-20130711163943/@comment-7418318-20130711172606

Maybe Tartaros uses it's associated minor dark guilds are "recruitment filters". The weak ones stay in the minor guilds and the strong ones get promoted to actual Tartaros members.

Remember that Minerva just recently joined (even that masked guy called her newbie), so it's not surprising she hasn't had the time yet to work her way up the ranks.

I wouldn't be surprised if we see her join Tartaros properly later on.