Kid is a skilled thief... at least we are led to believe she is, with nearly all the dialog in the game about her, though we never really get to confirm this...
My theory is this:
Lucca's orphanage burns down;
Kid is rescued, but then abandoned (unavoidably so);
Kid falls in with a group of thieves in order to survive;
Kid never feels at home with them... She is still "alone";
She leaves when the group refuses to help her go after an extremely high risk treasure of questionable existence and tenuous value to begin with. They also don't give a crap about her quest for vengeance one bit;
Kid leaves the mainland for El Nido;
Knowing she can't infiltrate a military stronghold like a general's manor alone (though never admitting it so easily) she looks for companions, and finds Serge;
Saving Serge from Karsh is partly due to her good (haha) nature, and partly to strong arm him into helping her;
That's the story as it stands.
Because it's difficult to organize large groups of people, especially when they are of questionable morals (thieves...) I doubt the Radical Dreamers is larger than a few... we'll say 10 members at most.
Kid may or may not have taken over the group at some point, and may or may not have been the one to name it. (She did name the Magical Dreamers however, either based on the name of the group she was in, or based on the name she gave the group, is unknowable.)
