Go to services,iis, make sure anonymous logon is allowed, you can disable this later,that will eliminate any user restricions causing the problem, but IMO, you would be better off with a 3rd party ftp server, I use SERV-U and its very good - a lot more configurable than iis.
If they squeeze olives to get olive oil, how do they get baby oil?
Also right click on the folder your sharing, if your using the default it should be FTProot. Then click on security and check to make sure the users you want access to that folder are listed there. Give them read or write access as you need.
Arguing on the internet is like running in the Special Olympics.