Dylan Boyd, Mohammad Omar Aly Hassan, Ziyad Yaghi, Hysen Sherifi and Anes Subasic face multiple felony charges related to allegations they conspired to attack the Marine base in Quantico, Va., and targets overseas.
Subasic rose in court Monday and said he was being framed by "crooks" in the government before entering his plea.
Accused ringleader Daniel Patrick Boyd pleaded guilty in February to charges of conspiracy to provide material support to terrorists and conspiracy to murder, kidnap, maim and injure persons in a foreign country.
Zakariya Boyd later pleaded guilty to a single count of conspiracy to provide material support to terrorists. Daniel Boyd is the father of Dylan and Zakariya. A jury trial is scheduled to start next month.