It is very difficult to extract only one MSER feature per person in your image. But tweaking the parameters
I = imread('people1.jpg');
I = rgb2gray(I);
regions = detectMSERFeatures(I,'RegionAreaRange',[3 3],'ThresholdDelta',2);
figure; imshow(I);
hold on;
plot(regions,'showEllipses',false);
[features2, valid_point] = extractFeatures(I,regions);
Hope this helps!