Led by head coach Neil Wood, the Reds topped Group F with five victories and just one defeat.
"It's been great to be back in the competition," Wood said. "We've had a couple of years out and you miss it, the whole experience of travelling away. They're quick trips where you're in one day, you play and you're straight back out of the country. It's really good to be back and it's been nice to get a good start to it as well."
Upcoming opponents Dortmund had to go through the play-offs after finishing second in Group C, where they faced tough opponents in Sporting, Ajax and Besiktas.