How to go to Yamingshan from Ximending? Is there a tour group we can join?
Both places are not far to each other and if by taxi/car, it's less than 30mins from Ximenting.
Or you can take the subway to Jiantan Station, and take the red-5 bus to Yangmingshan stop.
I trust there are day tours there and you may like to check with your hotel for that as some hotels do have a tour desk or they can do the booking for you.
Get quick answers from Yangmingshan National Park staff and past visitors.