(1) ON behalf of = as the authorized, official agent of (I am speaking ON behalf of the Smith family); (2) IN behalf of = for the benefit of (I'm donating $100 IN behalf of the homeless); = as a friend (I'm sure that I speak IN behalf of all posters when I thank the volunteers on this website for doing such a great job day in and day out).
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Some people use in and on interchangeably to convey both meanings, but this use should be avoided in careful speech.
Unfortunately, this is poor guidance; it does not reflect reality. That is my concern.