Given an RDS cluster with NLB, users connect and login to one of the servers at random.
But there is one program that's only available on one of the servers (licensing reasons).
Can we force a group of users to always be logged on to one specific server, and let everyone else have logins allocated normally?
Not with NLB. If you need them to log onto a specific RDS server you'll need to have them connect to the ip address or name of the specific server.