You can create ''Requirements Only'' users with the following process:
1) Log into your project in Quality Center and go to Project Customization (Tools -> Customize).
2) In Project Customization, go to 'Groups and Permissions' and specify a User Group that you will use for Requirements Only users. Make sure the desired users are ONLY a member of that User Group.
3) In Project Customization, go to 'Module Access' and select only the Requirements Module for your User Group from Step 2 (make sure the other modules are de-selected).
4) Now, when a user who is a member of that User Group logs into the project, only a Requirements license will be used.
You can use the same process for ''Defects Only'' users by selecting the Defects Module in Step 3.