edit users of assigned domain
A good start would be here: http://drupal.org/project/domain_roles - This allows you to segment roles by domain. So you could create different roles, then assign those roles to users and by way of other access modules (Content Access could be useful but I'm not certain how it handles user entities) you could then limit the users to only their domains.

FWIW, this used to be done in Drupal 6 by way of the unstable module: http://drupal.org/project/domain_user_edit - Could be worth contacting the maintainer and seeing what you can do to take it over for Drupal 7 and on.