Johnny's Jrs. are trainees who haven't debuted yet. They perform covers of their seniors songs on Shounen Club (Boy's Club) as "training". They also act as back up dancers for the groups that have already debuted.
JE's idols do acting, singing, hosting shows and modeling.
Apparently some of the guys don't know how to sing nor dance. Still they sale better than some other artists who are really gifted in singing.