Solidity exercise: adding members to contract

membership.sol

pragma solidity ^0.4.0;

contract Membership {

    address owner;
    address[] members;

    function Membership() {
        owner = msg.sender;
        signMeUp();
    }

    event NewMember(address newMember);

    function signMeUp() returns(string){
        if(amIMember()) {
            return "You are already a member!";
        } 
        members.push(msg.sender);
        NewMember(msg.sender);
        return "Welcome. You are in!";
    }

    function amIMember() returns(bool) {
        for(uint i = 0; i < members.length; i++) {
            if(members[i] == msg.sender) {
                return true;
            }
        }
        return false;
    }
}