once i've grouped the units, how do i ungroup them?

let's say i hv group 1(a,b,c,d,e) and group 2(f,g,h,i,j). i accidentally added 'f' into group 1. everytime i select group 1 i get (a,b,c,d,e,f) and group 2 (f,g,h,i,j). the unit 'f' will be part of both groups.

how do i capture screen shots while in game? print screen doesn't work for me.