Aileenah Horton, 18, entered the plea Friday in Pierce County Superior Court and faces a prison term of 86 to 114 months in prison at her sentencing, set for Sept. 6.
Barker was found beaten and strangled in the dirt outside a vacant home in Graham in September 2012.
When questioned by investigators, several of the suspects said they abducted and beat Barker because they believe he had molested a toddler, according to the statement of probable case. One suspect claimed Barker had also sexually assaulted a woman.
However, Pierce County Prosecutor Mark Lindquist said the suspects' claim of vigilante justice for the victim's alleged sex crimes may not be true as investigators have not found any evidence to support either claim.
After the beating, the suspects took Barker's wallet and split the cash, they told investigators. They also tied a red bandanna around his neck to "mark" him, the statement said.
Horton said she participated in the beating, but did not accompany the others to the home where the victim was abandoned.
Six others have been charged in the case. They are John Joseph Brown, 60; William Barry, 33; Angela King, 32; Justin Mahaffey, 23; and Joshua Stevens, 23. All have pleaded not guilty to charges of premeditated murder, kidnapping and burglary.