I'm set to join a large company in a couple of weeks. It has been a protracted process - almost 3 months between signing the offer letter and joining the company.
When I accepted the offer I was matched with a team and (as far as I know) they expect me to join them once I'm on-board. Three weeks before I start, a manager from another team at the same company found out, through a mutual acquaintance, that I'd be joining and wants to see if I'd be interested in joining his team instead.
I don't mind speaking to this manager and learning more but I have no intention of joining his team. It doesn't seem right, at the last minute, to ditch a team that has waited nearly 3 months for me, and may have missed out on other good new hires in the meanwhile. What is the most polite way for me to state upfront to this manager that I won't join his team, for the reasons I stated above?
EDIT (in response to comments): The first manager doesn't know about the second manager reaching out to me. The second manager hasn't offered me a role on his team yet - he merely wants to speak with me about the possibility. The second team's role doesn't constitute a separate job offer AFAIK - it'll still be the same title, salary etc. The role I applied and interviewed for was generic - I was matched with a team after completing the interviews.
EDIT2: The second manager texted me - this is outside of the regular process. HR is not aware of this second manager contacting me. HR is managing me through the logistics of finalizing a start date. Team matching was done just prior to finalizing the offer.
EDIT3: The company's standard selection process is a generic skills interview. If that goes well they match you with a team and prepare an offer.
Anyone want to clarify (with a comment perhaps) why this question is "opinion-based"? I'm asking for guidance on how to politely phrase an answer, not what answer I should give.