1) As many of others have said, allow earlier introduction through a staged process like the "Village Applicant". Every cabal can set different requirements, including powers for the base. Just as an example, an Scion applicant could summon a nightwalker (it is a level 15 or 20 spell anyways right?). You could also have this kind of thing handled by 51 Immortals.

2) There's also the flip side to creating more value to have lower level members....how about more incentive for lower level people to join cabals by having things like bonuses to kill each other? I'm just talking small bonus learning for killing another applicant to another cabal or raiding or even grouping with other cabal members (orc style). Of course it would follow the normal rules of "don't multikill the newbie", but it would promote raiding with the higher ranked cabal members with a benefit to the player. You could also make the outer fight better by having different strengths depending on who is fighting it....easier over all but special abilities based on it's opponent.