Or were only people from the present able to use the portal to the past?
shareSince he brought beef back from the past, though dead yet still organic, I would assume a lifeform could also come back with him. I cannot recall if the book touched upon this at all.
shareIn the book Sadie agrees to go back to the future with him after they save JFK. No indication is given that it isn't possible.
shareOn the show she agrees as well but she's shot by Oswald and dies instead of JFK.
Honestly, I don't think she would have made it though the rabbit hole. Like the YCM said she was ALWAYS going to die so no matter what Jake did, she would have died every time BEFORE going through the rabbit hole.