they started saying their group was "full" i think because the are sick of see people die and the heartbreaks that come with them when a person sad they become useless for a long period of time like carl
With Dale gone, they're 1 man down. Now they have plenty of room to take the kid in, fill in the spot, and watch him for awhile. I think the kid proved himself enough when he helped to save Shane instead of just running.