。
I'd like to join the night bus tour.
Literal
Night [gen-の] bus tour [to-に] want-to-join-[explanatory-のですが].
~たいのですが is a soft request/desire pattern — ending a statement with ~が leaves it open-ended, inviting a response or offering to help. Very polite and indirect, typical of service-counter interactions. 参加する ('participate, join') takes its target with に.