Those guys were either betrayed, sold, or forced by other people. So, when Cloud dealt with those nasty people, some of the people they rescued joined them.
Of course, not all the people they had rescued joined them but went to different bases or places where other survivors lived. Only the people that were willing to go to the north base with them were the ones left in the group.
At this time Cloud group had a total of 21 people, which was the total number after 4 people were gone because of the zombie rats.
According to Cloud they had been careful on their way back to the north base so they had less casualties and rarely lost someone. Along the way, they only lost 6 people before being chased by zombie rats because those people turned into zombies after being injured by stronger zombies.
And the time they died were all different, unlike when they ran away from zombies which caused them to lose 4 people immediately.
