Something i've been wondering, How did Harkon get all those vampires together? did he bite them and turned them same as he did to the player? or were they already vampires?

Meaning are all the members in the castle vampire lords? or just ordinary vampires?

I think Harkon turned them after Bal turned him.

They are probably from Harkon since they are vampire lords and not pure ones. Either Harkon or Valerica.