People use to spot each other for pullups. If you have someone who can help: Grab the pull up bar like your going to do a pullup, bend your legs at about a 90 degree angle and have them hold your feet like they are trying to keep you from falling, it will reduce a lot of the weight you're trying to pull up.
With all due respect: great intent, horrible idea. Never, never, never spot pull-ups by holding their feet. If they slip and fall, they fall on their knees or face.
Instead, hold them under their shoulder blades; just behind and below their armpits. That way, if they slip, they land on their feet. Plus, your assistance directly helps them on the way up as much as they need. Assisting the feet is several joints away and there are too many joints that can absorb the assistance.
Just some friendly knowledge from a CSCS (Certified Strength and Conditioning Specialist) from the NSCA (National Strength and Conditioning Association) to help prevent injury or legal action.