Get a user from a Person field and check if they are in a SharePoint group

If you have a Person field (people picker) in your SharePoint form, and need to get that value, check if they're in a group, and take an action based on that, you can use the following:

<script type="text/javascript">

function IsCurrentUserMemberOfGroup(groupName, pickerTitle, OnComplete) {

// get people picker
var peoplePicker = SPClientPeoplePicker.SPClientPeoplePickerDict[$('[title="' + pickerTitle + '"]')[0].id];

// get the user dictionary
    var users = peoplePicker.GetAllUserInfo();
   
    // get the user name
   var currentUserAccountName = users[0]['Key'];
       
      // get current context and web
      var currentContext = new SP.ClientContext.get_current();
        var currentWeb = currentContext.get_web();
                
        // get the user object
        var user = currentWeb.ensureUser(currentUserAccountName);
         
        // execute call to get user id
        currentContext.load(user);
    currentContext.executeQueryAsync(
        Function.createDelegate(null, OnUserSuccess), 
        Function.createDelegate(null, OnUserFailure)
   );

var groupUsers;

// Once we have the user, check if they're in the group
function OnUserSuccess(sender, args) {

// get all groups
var allGroups = currentWeb.get_siteGroups();
       currentContext.load(allGroups);

// get the group we're looking for
       var group = allGroups.getByName(groupName);
       currentContext.load(group);

// get the users of the group
       groupUsers = group.get_users();
       currentContext.load(groupUsers);

       currentContext.executeQueryAsync(OnGroupSuccess,OnGroupFailure);
}

// loop through all users in the group and look for our user
        function OnGroupSuccess(sender, args) {
            var userInGroup = false;
            var groupUserEnumerator = groupUsers.getEnumerator();
            while (groupUserEnumerator.moveNext()) {
                var groupUser = groupUserEnumerator.get_current();
                if (groupUser.get_id() == user.get_id()) {
                    userInGroup = true;
                    break;
                }
            }  
            OnComplete(userInGroup);
        }
        
        function OnUserFailure(sender, args) {
OnComplete(false);
}


        function OnGroupFailure(sender, args) {
            OnComplete(false);
        }    
}

function IsCurrentUserHasContribPerms() 
{
  IsCurrentUserMemberOfGroup("Leadership Team", "Nominee", function (isCurrentUserInGroup) {
    if(isCurrentUserInGroup)
    {
        alert('Leadership Team Members are not eligible to be nominated');
        $("input[value$='Save']").attr('disabled', true);
        $("span[id='Person']").css('color','red').css('font-weight','bold');
    }
    else {
    $("input[value$='Save']").attr('disabled', false); 
    $("span[id='Person']").css('color','black').css('font-weight','normal');
    }
  });
}

function SetOnChangeEvent() 
{
$('input[title="Nominee"]').blur(function(){
IsCurrentUserHasContribPerms();
});
}

$(document).ready(function(){ 

ExecuteOrDelayUntilScriptLoaded(SetOnChangeEvent, 'SP.js');

});
</script>