Assuming the grouped bars are centered at integer values along the x axis, the last line of code below places the x ticks at the halfway point between each integer that spans the x axis.
% Set up demo
y = [2 2 3; 2 5 6; 2 8 9; 2 11 12];
h = bar(y)
grid on
% Get axis handle if you don't have it already
ax = gca();
% Set x tick to 1/2 way between integers that span x axis
ax.XTick = (floor(min(xlim(ax))) : 1 : ceil(max(xlim(ax)))) + 0.5
If the x centers of the grouped bars were specified and are not centered at integer spacing, you can compute the halfway mark between each group like this.
% Set up demo
y = [2 2 3; 2 5 6; 2 8 9; 2 11 12];
h = bar([4,5,8,10],y)
grid on
% Get axis handle if you don't have it already
ax = gca();
% Set x tick to 1/2 way between bar groups
ax.XTick = unique([h.XData]) + [diff(unique([h.XData]))/2, inf];
