function findUserIndex(sUserName) {
		for (var i = 0; i < aUsers.length; i++) {
			if (aUsers[i].UserName === sUserName) {
				return i;
			}
		}
		return -1;
	}
Looks for a user with a given user name and returns its index in the user array.
@param {string} sUserName - the user name to look for.
@returns {int} index of that user in the array, or -1 if the user was not found.