On Behalf of
* Most common: "On behalf of" is the more common and versatile phrase. It's used when you are speaking or acting for someone else, but you are not necessarily taking their place.
* Example: "I would like to thank you on behalf of the entire team for your hard work." (Here, you are speaking for the team, but you are not acting as the team itself.)
In Behalf of
* Less common: "In behalf of" is less common and can sometimes sound archaic. It's used when you are specifically acting as a substitute or representative for someone else, taking their place.
* Example: "I am writing this letter in behalf of my client who is unable to attend the meeting." (Here, you are acting as a proxy for your client.)
In practice, most people use "on behalf of" in all situations, and it's generally considered correct. You are unlikely to go wrong using "on behalf of". However, if you want to emphasize that you are acting as a direct substitute for someone, "in behalf of" can be used.
Here's a helpful tip: If you're unsure which to use, always go with "on behalf of". It's the safer choice.