Are there sleeper compartments in ÖBB Nightjet trains for small groups?

Yes. Two adjacent sleeper compartments can be converted via a connecting door into a suite for up to six people. Combine either two Economy compartments or one Economy and one Deluxe compartment. These variants cannot currently be booked online, but only at the DB Travel Centre, at a travel agency with a DB licence or via the Deutsche Bahn service number on  +49 30 2970 (charges depend on provider).