I realize this is old but if anyone stumbles upon... I use the DataOnTap powershell add-on to script auto creation of Home dir shares in NetApp. (Automaticaly works for employee moving to new shares as well, though I don't migrate data automatically). My script is here:
By default I poll the filer for which volume matching Users* has the most available free space, but I added a flag in to set that will switch it to use a regex lookup matching the first letter to a pattern of [0-f] to users1 [g-m] to users2 [n-s] to users3 etc.,, so Jdoe would be would land on volume users2 for example. This is what you're asking for... Just set the flag at the top to true (in the parameters, which are all commented to explain), and then around line 269 manually modify the regex patterns to match your use case, and more importantly to match the actual volume names needed.
I run this with a scheduled task every 5 minutes, so after an account is created in Active Directory by the helpdesk, the directory and share itself follows suit almost immediately.
This is explicitly creating shares for the user with the specified name, which generally is the NT ID but does not have to be. If you're using the built in NT login ID matching you could eliminate about 1/2 the script or just comment out the add-nashare line that actually tries to create the share.