GrantAccess.cs

        /// <summary>
        /// Contains the data that is needed to grant a security principal (user or team) access to the specified record.
        /// </summary>
        /// <param name="orgService"></param>
        /// <param name="sharedRecord">The entity that is the target of the request to grant access. Required.</param>
        /// <param name="sharedUser">The team or user that is granted access to the target record.</param>
        private void GrantAccess(IOrganizationService orgService, EntityReference sharedRecord, EntityReference sharedUser)
        {
            var grantAccessRequest = new GrantAccessRequest
            {
                PrincipalAccess = new PrincipalAccess
                {
                    AccessMask = AccessRights.ReadAccess | AccessRights.WriteAccess | AccessRights.AppendToAccess,
                    Principal = sharedUser
                },
                Target = sharedRecord
            };
            orgService.Execute(grantAccessRequest);
        }

        /// <summary>
        /// Contains the data that is needed to replace the access rights on the target record for the specified security principal (user or team).
        /// </summary>
        /// <param name="orgService"></param>
        /// <param name="sharedRecord">Target record for which you want to revoke access. Required.</param>
        /// <param name="sharedUser">A security principal (team or user) whose access you want to revoke. Required.</param>
        private void RevokeAccess(IOrganizationService orgService, EntityReference sharedRecord, EntityReference sharedUser)
        {
            var revokeUserAccessReq = new RevokeAccessRequest
            {
                Revokee = sharedUser,
                Target = sharedRecord
            };
            orgService.Execute(revokeUserAccessReq);
        }

        /// <summary>
        /// Contains the data that is needed to retrieve all security principals (users or teams) that have access to, and access rights for, the specified record.
        /// </summary>
        /// <param name="orgService"></param>
        /// <param name="sharedRecord">Records you want to retrieve security principals (users and teams) and their access rights</param>

        private static void RetrieveSharedUsers(IOrganizationService orgService, EntityReference sharedRecord)
        {
            var accessRequest = new RetrieveSharedPrincipalsAndAccessRequest
            {
                Target = sharedRecord
            };

            var accessResponse = (RetrieveSharedPrincipalsAndAccessResponse)orgService.Execute(accessRequest);
        }