Seeing the parallel structure of this sentence will help. The sentence is grammatically correct, and the omission of the second “to” is legal, but for clarity I am spelling it out.
Attendance is limited to maintain an intimate setting and to foster dialogue among all participants.
Restated:
Attendance is limited = We are keeping the attendance low, or small
I don’t like this use of “limited,” although it is very common. In reality anything that isn’t infinite is limited, and the limit may be very high. They could limit the attendance to one million and it would still technically be limited. But people use it to mean that there’s a low limit—that is, not many can attend.
to maintain an intimate setting = in order to maintain (for the purpose of maintaining) a feeling of closeness (Intimate, sense 4) among the participants so they will feel comfortable speaking in the group (setting, sense 2)
and to foster dialogue among all participants. = and in order to encourage (foster, sense 1) speech and discussion among all those who attend.