Book through nightjet.com (add Eurail as a discount)
It's way better to book directly on the companies websites when possible. That way you have no booking fee + more choices
Night trains quickly fill up so it's best to book as soon as you know your travel dates.
If you're five book for 6 people (add a fictive 6th person) if you want to get the whole compartment. There are also sleepers options (2 or 3 people) but more expensive
Reservations are optional in Germany, Austria and many other countries. You can book them if you'd like on tickets.oebb.at (add Eurail as a discount) : 3€ 2nd class, free with 1st class pass
For multiple journeys in or near Germany : 4.50€ 2nd class / 5.90€ 1st class (no matter how many trains)
Of course this doesn't apply to couchettes or sleepers, for them use nightjet.com or tickets.oebb.at