I am using the admin generator for the sfGuardUser module. The edit portion of the generator.yml file looks like this: 
edit:
  title: Editing User "%%username%%"
  display:
    "User":  [first_name, last_name, email_address, username, password, password_again]
    "Permissions and groups": [is_active, groups_list, sites_list]
Now, not every user will have access to this form, only site administrators allowing site administrators to create and update their own users. There is a many-to-many relation between User and Site. Each site administrator is also a user and as such has a set of associated sites. 
I would like sites_list to not show ALL sites, but rather, only the sites the site administrator is associated with thereby ensuring that a site administrator cannot put one of her own users into a site the administrator is not associated with.
It seems to me I need to replace sites_list with something else to do this, but I do not know where and how to make this change.
                        
The only way that I think to to this is by changing the sites_lists widget from the Autogenerated Form. In your case, for example, you could do something like: