A Quorum node's keypair is generated using Constellation. Depending on which version you have, you run either of the following to generate tm.key (private key) and tm.pub (public key) files. You can just hit return when prompted to set a password unless you really want one.
The public key is the one you need to use in the
If you are interested in exploring Quorum beyond 7-nodes, I have an example that spins up as many nodes as you request in separate Docker containers, and generates all of the enodes, keys, genesis file etc. You can find it here: https://github.com/ConsenSys/quorum-docker-Nnodes
See the exposition of the setup file for all the details about key generation, config files etc.