There are two things to be done here:
1) to make sure the user has access rights to the folder where the files are;
2) to make sure the user is allowed to log on remotely to the machine.

1) Right-click the FTP folder (Inetpub\ftproot\username) and select Properties, go to the Security tab and add the account to the list, then select full permissions for it.
2) In Administrative Tools, run Local Security Policy (or Start > Run > secpol.msc). Under Local Policies > User Rights Assignment, open the key (Allow) log on locally and add the username to the list. You might also need to add it to "Access this computer from the network", I'm not sure.

For easier maintenance, I'd recommend creating a group called "FTPAccess". To give someone access to the FTP, you simply add the person's account to this group. There are tricks you can use to restrict access to each person's folder as a group, without doing manual settings for each account.