What do I do if somebody asks me to join his accusation of my colleague of academic misconduct?
Delete the email (or move it to one of your folders where you keep old ones).

From what you describe, you have no business joining the complaint since you have no complaint again colleague A. It is, in other words, not something that actually affects you: someone has beef with colleague A, and that is their business, nor yours. Worse, if you did get involved -- even if just by writing back to B --, there is nothing for you to gain, just collegiality to lose. It's neither worth your time nor your reputation to act any further than reading, then ignoring the email.

If anyone ever asks you whether you got the email, just tersely say "Yes, I got it; it was none of my business, so I ignored it."