The Externals are in a sense "immortal" so one can only guess when they banded together, whether there was different incarnations of the team and such other things but one must assume they have been around for awhile.
The latest incarnation of the Externals was mainly comprised of eight members, the most dominant being the five males that made up the High Lords.
The High Lords' goal was to play power games by manipulating everything from behind the scenes. They were Saul, Absalom, Gideon, Nicodemus and Burke. They also employed the sixth male External, Crule as their enforcer and assassin.
However, the two female members of the team, Candra and Selene, kept to their own schemes and were not a part of the gathering of the High Lords.
In more recent days, Cannonball was declared an External by Cable. Yet this is denied by Selene and has not been delved into anymore than this, leaving some to question it.
Selene also denies that Apocalypse is an External because of the fact that she sees the Celestial technology being the thing that keeps him alive, even though he only found it several millennia after his birth.
The Externals no longer exist thanks to a series of events that wiped them out. Nicodemus and Burke were the first to die, after contracting the Legacy Virus and dying from it.
Candra was killed by Cyclops, as she so foolishly placed her life force in a gem. Crule, Saul, Gideon and Absalom were all drained of their life force by Selene.
Now the Externals have disbanded as Selene's whereabouts are unknown, though she has taken Wither to be her groom. Apocalypse has been taken by the Celestials as payment for him using their technology. And Cannonball still remains unconnected to the team, likely having forgotten it ever existed.
Earth-4935 In the world of the Askani, the Externals were all killed by Apocalypse who then rose to power over the Earth. He remained in this position up until the 39th century.