Hi @sureshp89882164 

You can also achieve this functionality with the help of Onload Client Script and Client Callable Script Include :

OnLoad Client Script:

function onLoad() {
    if (g_form.isNewRecord()) {
        g_form.setReadOnly('assigned_to', false);
        return;
    }
    var assignmentGroup = g_form.getValue('assignment_group');
    var userSysId = g_user.userID;
    if (assignmentGroup) {
        var ga = new GlideAjax('CheckUserAssignmentGroup');
        ga.addParam('sysparm_name', 'isUserInAssignmentGroup');
        ga.addParam('sysparm_user', userSysId);
        ga.addParam('sysparm_group', assignmentGroup);
        ga.getXMLAnswer(function(response) {
            if (response === 'true') {
                g_form.setReadOnly('assigned_to', false);
            } else {
                g_form.setReadOnly('assigned_to', true);
            }
        });
    } else {
        g_form.setReadOnly('assigned_to', true);
    }
}

Script Include :

var CheckUserAssignmentGroup = Class.create();
CheckUserAssignmentGroup.prototype = Object.extendsObject(AbstractAjaxProcessor, {

    isUserInAssignmentGroup: function() {
        var userSysId = this.getParameter('sysparm_user');
        var groupSysId = this.getParameter('sysparm_group');

        var healthGroupSysId = '24e93416c35c6a10f99dba2ed4013147'; //Replace your Group Sysid
        var gr = new GlideRecord('sys_user_grmember');
        gr.addQuery('user', userSysId);
        var groupQuery = gr.addQuery('group', groupSysId);
        groupQuery.addOrCondition('group', healthGroupSysId);
        gr.query();
        if (gr.next())
            return 'true';
        else
            return 'false';
    }

});

change your table to problem.

If my response helped please mark it correct.