I don't understand adjacent? Would a line of 4 infantry count as a "group" of adjacent infantry? As long as you can follow a path, is it adjacent, or do they need to be 4 bunched up units, literally all adjacent to every other unit in the group.
Adjacent means one hex is touching at least one other hex. It can be a straight line, a group, some jagged edge. Basically, think of any Tetris shape - if it were applied to hexes - and it would be legit.