Originally Posted by Hawk
They are going to have to show how she is brought back or else it won't make sense to viewers. Speculation is Beric Dondarrion sacrifices himself to give his ability to escape death to her which is how she ends up leading Beric's band (at least it appears she is leading them when she kills that Frey guy in the epilogue). Maybe we'll get something explicit in the show because it won't make a lot of sense for her to just pop up and be alive again.
You haven't read past SoS? It's explained later how she is back, and she is undoubtedly leading the Brotherhood without Banners.