The transition to IM/P is a bit confusing. But the thing to remember is that even though the location of the "licensing" and the location of creating the profiles differ, everything else is identical between CUPS and IM/P.
You still perform node assignment on the IM/P server webpage itself directly as you always have. This is done under System -> Cluster Topology. You can set auto-node assignment under the Cisco UP Sync Agent Service parameters. Options consist of "balanced", "Active-Standby", or "None". All of which should be fairly self explainatory.
For the permission to allow this on the IM/P server we actually pull the permissions from Call Manager for end users. You can only adjust the roles (atleast the last time I checked) for Application users, so this means you cannot have an end user with only permissions to IM/P and not CUCM. Having said this I'm not sure if there's a possible subset of permissions on CUCM which will allow access to that page with an end users. I do know standard CCM Super Users will work just fine.