Windows 7: finding network user

Hello there



I have about 4 different computers on the same network, and I'm trying to allow UserXX from computerXX access and modify files on this machine. When you go and select a file-rightclick-Share, then Advanced, you can "Add" users. Under Browse for the location, you are (I guess..?) supposed to see everyone on the network, at least thats what it appears like. However, the only users I'm able to pick from are the local ones, and I'm not able to type another computer name, or anything. When I try to modify a file from another one of my machines, it doesn't recognize that user and blocks me.

Help!!
any input?