Created using browser-solidity: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://ethereum.github.io/browser-solidity/#version=soljson-v0.4.15+commit.bbb8e64f.js&optimize=undefined&gist=3a7d92ae197b19c82b023d90ab6bbb56

UserManager.sol

pragma solidity ^0.4.11;

contract UserManager {
  address   public owner;
  address[] public users;
  mapping(address => uint) userIndexes;
  
  modifier onlyOwner {
    require(msg.sender == owner);
    _;
  }

  function numUsers () constant public returns(uint) {
    return users.length;
  }
  
  function getUsers () constant public returns(address[]) {
    return users;
  }
  
  function hasUser (address searchUser) constant public returns(bool) {
    if (users.length > 0) {
      uint searchIndex = userIndexes[searchUser];
      address foundUser = users[searchIndex];
      return searchUser == foundUser;
    } else {
      return false;
    }
  }

  function UserManager () {
    owner = msg.sender;
  }
  
  function addUser (address newUser) onlyOwner {
    require(!hasUser(newUser));
    userIndexes[newUser] = users.push(newUser) - 1;
  }
  
  function removeUser (address oldUser) onlyOwner {
    require(hasUser(oldUser));
    uint oldUserIndex = userIndexes[oldUser];   
    address lastUser = users[users.length - 1]; // copy the last address in the user array
    users[oldUserIndex] = lastUser;             // replace the old address with the last address
    delete users[oldUserIndex];                 // delete the last address from the array
    userIndexes[oldUser] = 0;                   // map old address to index 0
    users.length--;                             // decrement the length of the user array
  }
}