User blog comment:FairyRaven/Fairy Tail Chapter 346: Retrogression Magic, Review and Commentary/@comment-7418318-20130802103725

Minerva might have meant that she joined the strongest guild she was able to. Surely a guild as secretive as Tartaros has some pretty stric regulations regarding recruiting. Most likely any potential recruits first have to proof themselfs by serving for a time in one of their associated "lesser" Dark Guilds, which would also fit in perfectly with Minerva's commenting about Succubus Eye being just a stepping stone and her going higher (shouldn't that be lower, considering she's talking about the underworld here? :P ).