[Ethereum] How to send from multiple accounts to the one account in the contract

In order to send Ether, you need to pay a transaction fee of 21k gas (more if you are sending to or from a contract-based wallet).

Mist rounds some Ether values, so it is very difficult to send exactly the amount in your wallet. To send roughly the amount you want, just lower the transaction value by 0.0001 ETH or so, or reduce the transaction fee by sliding the "select fee" slider left.

If you really want precision, you can use the command line client that is packaged with Mist. Open up your command line, and cd into the ./resources/node/geth/ directory. Then run

./geth --exec "eth.sendTransaction({from:'0xADDRESS',to:'0xRECIPIENT',value:eth.getBalance('0xADDRESS')-21000*50000000000-1,gas:21000,gasPrice:50000000000})" --unlock '0xADDRESS' console

Replace both instances of ADDRESS and RECIPIENT with your address and the recipient's address respectively. Leave the quotation marks.

If I understand your question correctly you want to be able to see who has deposited Eth to your contract address. This is what event logs are for.

(1) Create a contract where there is an event every time there is a transaction. e.g something like:

contract someContract {   

    address public owner;

    // Set the owner of the contract to be the creator of the contract i.e. you
    function someContract() {
        owner = msg.sender;
    } 

    // This is an event 
    event DepositMade(address _from, uint value);
    event WithdrawalMade(address _to, uint value);

    //Catch all function 
    function() {
        // generate an event when someone sends you Eth
        if (msg.value > 0)
            DepositMade(msg.sender, msg.value);
    }

    // Only the owner of the site can withdraw Eth
    modifier admin { if (msg.sender == owner) _ }

    function withdraw(uint amount, address recipient) admin {
        if(recipient.send(amount))
            WithdrawalMade(msg.sender, msg.value);
        else throw;
    }
}

The important bits are defining an event type event DepositMade(address _from, uint value) and generating an event when something happens DepositMade(msg.sender, msg.value);these events are stored in the event log associated with the address of the deployed contract instance.

(2) You retrieve the events on this contract using rpc eth_newFilter or web3.eth.filter e.g something like:

var filter = web3.eth.filter({fromBlock:0, toBlock: 'latest', address: contractAddress, 'topics':['0x' + web3.sha3('DepositMade(hexstring,uint256)')]});
filter.watch(function(error, result) {
   if(!error) console.log(result);
})