intro: rename mpibgrid to hpc-users

introduction/login.rst

 Access and Login
 ================
 
-To gain access your account needs to be added to the **mpibgrid** access group.
+To gain access your account needs to be added to the **hpc-users** access group.
 Drop us an E-Mail if you would like to join, there are no restrictions, we
 simply would like to talk to you beforehand.