Try to actived technical mode (V8) / developers mode (V9)

Under setting, group menu will show up.

Dress,

Please follow the steps:

1) Create your desired group "YOU_GROUP"

2) go to access rights and give res.users and give create/Edit/write/delete access for it

3) Now go to Settings -> Technical -> Security -> Access Controls List , and search for res.users object, you will get the list of all the access rights set for res.users object with group names

4) For all the groups other than your created "YOU_GROUP", remove create right access., and if you want to restrict read access also, remove read right too....

5) Now no one other than "YOU_GROUP" member will be allowed to create users...

6) One more thing, be sure of  "YOU_GROUP" doesn't inherit any other group.

Regards!!