Unlike Jude where the heretics are people who slipped in from outside, Peter presents them as false teachers who are part of the Christian community. They are members in the church who then begin to introduce new and strange doctrine—doctrine that comes from outside and not doctrine that comes from prophetic writings or the apostolic word.1
1 But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction.