There were four soldiers who went to the residence, knowing that the plan was for the girl to be raped. They are referred to in the affidavit as SOI2, SOI3, Green and KP1. You can read the affidavit for Green's arrest here. Page 6 lays out the events and players.
Amid the more disgusting details, provided by SOI2 and SOI3 who have cooperated with authorities: They go to the house, SOI3 stays in front on guard, the other three go in the house. KP1 smacks the girl down in the living room, Green goes in the bedroom, shoots and kills her three family members. SOI3, hearing the shots, comes in the house. Green comes back out to the living room where Green and KP1 rape the girl, after which, Green shoots her and kills her. SOI2 tells SOI3 to get rid of the AK-47 Green used to kill them all.
The military has not yet released the names of the five men charged, but I'm assuming three of the four charged with the rape and murders are SOI2, SOI3 and KP1, who went to the house with Green.
Re-reading the affidavit, the fourth could be SOI1, who agreed to monitor the radio and provide one of his weapons to the others, knowing they were headed from the checkpoint where all were on duty to the house where they intended to rape the girl.
The fifth soldier charged with dereliction of duty for failing to report the crime could be SOI4, who discussed it with SOI1 afterwards and was made aware of what happened and who was involved. Or it could be SOI5 who was on duty at the checkpoint on 3/12/06 when an Iraqi reported the crime and who went the house and saw the crime scene but didn't report it.
What's up with all the initials? SOI is Source of Information. SOI1 would be the first soldier who came forward to talk. SOI2 would be the second and SOI3 the third, etc. They voluntarily told their stories to investigators which included their version of the rape and killings.
KP1 means "Known Participant 1." He, along with Green, allegedly raped the young girl before Green shot her. I doubt he has cooperated because if he had, I think he'd be referred to an SOI not a KP.
As for why Green is charged in federal court while the others are charged in military court, I'm assuming it's because Green had already been discharged by the time charges were filed while the others are still on active duty.