Domain administrator role that can only see users of that domain
Have you tried the Domain Admin module?  Looks like there's a bit of stuff in there.  It's a shame that a few of the features listed are currently missing for Drupal 7 though.


  Domain Admin adds more granular permissions to the Domain Access module and several of its sub-modules.
  
  Currently the module allows the site admin to delegate the following tasks:
  
  
  Access the domain overview page - delegated user will only be able to see the domains assigned to him/her (not there in D7 yet)
  Editing of domain record, domain settings and domain theme
  Administer primary and secondary menus (also navigation in D6)
  Translate interface into the domain language (wont be ported to D7 - use i10n_client instead)
  View, edit and delete all unpublished/own unpublished nodes on domain (needs to be tested in D7)
  Administer book outline/ rearrange child pages of nodes on domain (requires book module)
  View webform results on domain (requires webform module)