If you want to access to assign a user or a group to access NSX, you need to assign rights for the user/group to vCenter first.
Then only after that NSX can give permission to the user, as per this doc NSX assign a vCenter User: Assign a Role to a vCenter User
If the user does not exist or has not given a permission to vCenter, then you will not be able to give NSX permission to the user.
A minimum Read-Only access to vCenter is required.
If you want to give permission to a group, then you can review this doc: Group-Based Role AssignmentsBayu Wibowo | VCIX6-DCV/NV1 person found this helpful
Author of VMware NSX Cookbook http://bit.ly/NSXCookbook
https://nz.linkedin.com/in/bayupw | twitter @bayupw