OneStickShortOfaBundle -- If you created a dedicated user for the TM share and didn't specify a group when doing so, the group name for that user should be the same as the user's name. Ie, I create a user account Alex, and don't specify the group, the default group of Alex will be created. So, in this case the new user would be Alex and he would be in the group Alex. If you can provide info on how you have created your TM share and how users authenticate to it would be appreciated... Best of luck in the mean time...
I figured out my own problem.... I created a dedicated user account for my TM share. I originally had permissions of the ZFS dataset with that user set for read/write, and ALSO had a password for the AFP share itself. Apparently TM didn't like the AFP share having a password. As soon as I removed that share password, I was still prompted to enter the user credentials to access the ZFS dataset and boom, it worked. Perhaps there was a change made somewhere along the line.... I used to be able to do both, meaning, the user account for the ZFS share, and a separate share password for the AFP TM share.....