You might want to check out HOG detector for OpenCV which tries to detect standing pedestrians, although you're going to get a lot of false positives and missed detections from it. Also, it seems they only have documentation on the website for the GPU implementation (i.e. using CUDA) and you'll have to look elsewhere for examples of the CPU implementation.
Another possibility to check out is the OpenCV face detector. Some ROS packages that use this are face_detector (uses stereo images though) and pi_face_tracker (for Groovy though).
In general, detecting people from a moving robot with a monocular camera is still a very difficult and open problem. If your camera is stationary then the problem can be simplified through background subtraction but I don't have any references for that.
For help with using OpenCV in ROS, checkout the vision_opencv package.
Edit: I uploaded my implementation of both of these in a ROS package on Github: https://github.com/angusleigh/hog_haa...