I have written a Javascript code which allows to hide Custom Field from all users except groups/roles defined in script.

Be aware that this is just a visual way to hide field. It will be possible to see field in HTML source of page.

Installation: paste it in Announcement Banner and set variables based on your environment.

<script type="text/javascript">
  /*	Karlis Rozenbergs, 04.2016

    Script allows to hide custom field for all users except groups/roles defined below. 
    By default project admin and users mentioned in following field are allowed to see hidden field: customfield_10865 (Confirmed by)
  */

  //Sets field to hide
  var hidefield = "#customfield_12345-val";

  //Sets groups which ar allowed to see defined customfield
  var allowedgroups = ['jira-administrators'];

  //Sets projects where to hide customfield. If empty, all projects are selected.
  //example: var projects = ['TEST', 'TEST1'];
  var projects = ['TEST1', 'TEST2', 'TEST3'];

  //Sets issue type where to hide customfield. If empty, all issue types are selected.
  //example: var issuetype = ['19', '3'];
  var issuetype = ['1', '2', '3'];

  //Sets roles which ar allowed to see defined customfield
  var allowedroles = ['Developers', 'Users'];


  //Gets current project key
  issueKey = AJS.$("meta[name='ajs-issue-key']").attr("content")

  var project = getProjectKey(issueKey);

  var flag = 0;
  var flag1 = 0;

  //Check if issue is in defined project
  for (l = 0; l < projects.length; l++){
    if (project == projects[l]) { flag = 1; }
  }

  if (flag == 1) {
    
    var issuety = getIssueType(issueKey);
  
    //Check if issue is in defined issue type
    for (k = 0; k < issuetype.length; k++){
      if (issuety == issuetype[k]) { flag1 = 1; }
    }
    
    if (flag1 == 1) {

      var user = getCurrentUserName();
      var aggrName = getIssueAggr(issueKey);
      var aggrName1 = getIssueAggr1(issueKey);
      var flag2 = 0;
      var flag3 = 0;

      //Gets urls of allowed roles
      var ProjRoles = [];
      for (k = 0; k < allowedroles.length; k++){
        ProjRoles.push(getProjectRoles(project, allowedroles[k]));
      }

      //Gets allowed users
      var ProjectRoleUser = [];
      for (l = 0; l < ProjRoles.length; l++){
        ProjectRoleUser.push.apply(ProjectRoleUser,getProjectRoleUser(ProjRoles[l]));
      }

      var allowedobjects= [];
      allowedobjects.push.apply(allowedobjects, ProjectRoleUser);
      allowedobjects.push.apply(allowedobjects, aggrName);
      allowedobjects.push.apply(allowedobjects, aggrName1);

      //Checks if current user is in array
      for (i = 0; i < allowedobjects.length; i++){
        if (user == allowedobjects[i]) { flag2 = 1; }
      }

      //Checks if current user is in defined groups
      for (j = 0; j < allowedgroups.length; j++){
        if (isUserInGroup(user, allowedgroups[j])){ flag3 = 1; }
      }


      //Checks if all projects and issue types are allowed
      if (projects.length == 0) { flag = 1;}

      //Checks if all projects and issue types are allowed
      if (issuetype.length == 0) { flag1 = 1;}

      //Hides defined field
      if (flag2 == 0 && flag3 == 0 && flag == 1 && flag1 == 1) {
        (function($) {
            AJS.toInit(function(){
            AJS.$(hidefield).parent().hide();
            AJS.$(hidefield).parent().value = "";
            });

            JIRA.bind(JIRA.Events.NEW_CONTENT_ADDED, function (e, context) {
            AJS.$(hidefield).parent().hide();
            AJS.$(hidefield).parent().value = "";
            });
            })(AJS.$);
      }
    }
  }
    
  function getCurrentUserName()
  {
    var user;
     AJS.$.ajax({
      url: "/rest/gadget/1.0/currentUser",
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        user = data.username;
      } 
     });
     return user;
  }

  function getGroups(user)
  {
    var groups;
     AJS.$.ajax({
      url: "/rest/api/2/user?username="+user+"&expand=groups",
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        groups = data.groups.items;
      } 
     });
     return groups;
  }

  function isUserInGroup(user, group){
    var groups = getGroups(user);
    for (i = 0; i < groups.length; i++){
      if (groups[i].name == group){
        return true;
      }
    }
    return false;
  }

  function getProjectKey(issueKey)
  {
    var projKey;
     AJS.$.ajax({
      url: "/rest/api/2/issue/"+issueKey,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        projKey = data.fields.project.key;
      } 
     });
     return projKey;
  }


  function getIssueAggr(issueKey)
  {
    var aggrName = [];
     AJS.$.ajax({
      url: "/rest/api/2/issue/"+issueKey,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        if (data.fields.customfield_10865) {
          for (var i=0; i < data.fields.customfield_10865.length; i++){
            aggrName.push(data.fields.customfield_10865[i].name);
          }
        }
      } 
     });
     return aggrName;
  }

  function getIssueAggr1(issueKey)
  {
    var aggrName1 = [];
     AJS.$.ajax({
      url: "/rest/api/2/issue/"+issueKey,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        if (data.fields.customfield_10864) {
          for (var i=0; i < data.fields.customfield_10864.length; i++){
            aggrName1.push(data.fields.customfield_10864[i].name);
          }
        }
      } 
     });
     return aggrName1;
  }

  function getIssueType(issueKey)
  {
    var issuety;
     AJS.$.ajax({
      url: "/rest/api/2/issue/"+issueKey,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        issuety = data.fields.issuetype.id;
        
      }
     });
     return issuety;
  }

  function getProjectRoles(project, role)
  {
    var ProjRoles;
     AJS.$.ajax({
      url: "/rest/api/2/project/"+project,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        ProjRoles = data.roles[role];
      } 
     });
     return ProjRoles;
  }

  function getProjectRoleUser(RoleUrl)
  {
    var ProjRolesUser = [];
     AJS.$.ajax({
      url: RoleUrl,
      type: 'get',
      dataType: 'json',
      async: false,
      success: function(data) {
        if (data.actors) {
          for (var i=0; i < data.actors.length; i++){
            ProjRolesUser.push(data.actors[i].name);
          }
        }
      } 
     });
     return ProjRolesUser;
  }

</script>