Sakura could also be the one. If she's under some kind of mind control or if she misinterprets Naruto's actions as being dangerous to the team or the village. She might side with the wrong people and betray Naruto. For instance, if Orochimaru tricks her into believing Naruto is a threat, she could turn on him in the Forest of Death.
It could even be their sensei. If the sensei has some hidden motives or is being blackmailed. He could order the team to betray Naruto. In the Forest of Death, where danger lurks everywhere, this betrayal would be especially cruel for Naruto as he would expect his teacher to be on his side and protect him.
Sasuke could be a likely candidate. He's always been somewhat jealous of Naruto's growth and popularity. In the stress of the Forest of Death, he might let that jealousy get the better of him and betray Naruto. Maybe he thinks that by getting rid of Naruto, he can be the only strong one in the team.