Please adjust the function. comments inline.

function getUserId(email) {
	var user_sys_id = "-1"; //No user found
	var getSysId = new GlideRecord('sys_user');
	getSysId.addQuery('user_name',email); //check if you are looking up in the correct field with email.
	getSysId.setLimit(1);
	getSysId.query();
	if(getSysId.next()){
		user_sys_id = getSysId.sys_id.toString();
	}
	return user_sys_id;
}