I think you are
The moon! - and in particular the dark side.
Part of me shines bright at night,
though me and my scars you cannot see,
not invisible but out of sight,
the light is never facing me.
The moon has a bright side that shines (the side facing the sun) and a dark side which the light never faces. The dark side of the moon cannot be seen by us. The scars are craters on the surface.
My other face likes watching you,
you know me, not a stranger,
I've always had one job
of protecting you from danger.
The other face of the moon faces earth, and we all know the moon. The dark side of the moon protects us from danger by shielding from meteors perhaps?
I always stay but not the same,
I rarely see my owner,
my members were my enemies,
I'll always stay a loner.
The moon is always there but has different phases. The dark side of the moon rarely can see the earth. There is only one moon and there won't be another. Thanks @Stiv for the members part: members of the dark side are the bad guys in star wars.