This is what I see happening in the source code of the Protectpage.aspx, it loops over all Roles and then checks if the role is allowed for the document. You can use that also I think, just use the following line to check if the document is protected by a certain role

Access.IsProtectedByMembershipRole(int.Parse(helper.Request("id")), role)

When you have all the selected roles use Roles.GetUsersInRole("Your RoleName") to get the users assigned to that role.

 Below the source code in Umbraco, hope the formattings stays okay after submit

 
// Load up membergrouops
_memberGroups.ID = 
"Membergroups";
_memberGroups.Width = 175;
 
string selectedGroups = "";
 
string[] _roles = Roles.GetAllRoles();
 
if (_roles.Length > 0)
{
 
foreach (string role in _roles)
{
 
ListItem li = new ListItem(role, role);
 
if (!IsPostBack)
{
 
if (cms.businesslogic.web.Access.IsProtectedByMembershipRole(int.Parse(helper.Request("nodeid")), role))
selectedGroups += role + 
",";
}
_memberGroups.Items.Add(li);
}
}
 
else
{
p_noGroupsFound.Visible = 
true;
rb_advanced.Enabled = 
false;
}
_memberGroups.Value = selectedGroups;
groupsSelector.Controls.Add(_memberGroups);

 Cheers,

Richard