Delegation of User Creation 

1. Edit the Project Permission Scheme
2. Grant wanted users to the permission Create Users.
    
3. Let these powered users to access to the feature

User Creation from Project Roles screen

As soon as a user is granted the Create User Permission, it will be see an additional button in the Project Role screen.

1. Click on Create User,

   With JIRA 6.3.X to JIRA 6.4.X

   With JIRA 7.0.X to OVER

2. Fill User Details ..
   

3. And perform the User Creation.

   Take care, that powered users will be allowed only to create new users, not to update them.