This is one reason I'm not a fan of AL. No way to legally and honestly jump in at higher levels.
But to answer your question, it would be best to have some story to explain your presence. Perhaps the DM could work you in as the subject of an escort mission, to give the players an incentive for protecting you. Or, going the otherway, you are a retainer who somehow gets dragged into dangerous situation above your pay grade. I think you'd do better with a supporting character class like bard or cleric, where at least you can contribute with buffs and minor healing. You just wont survive melee combat at this level, so you'll likely be bored or at least feel impotent playing a martial archetype. Rogue is fine, as it will help you hide, but how will you actively contribute. Hopefully the DC levels on traps and locks are not set too high. Also, if there is another rogue in the party but one that is 8th level, you'll likely just be the backup or apprentice to them.