Network Permissions and accounts

Hello, sorry if i cant explain too much but im in a hurry, I have a very small network of 3+ pcs, and i have some shared folders on each machine, my question is how can i let only some users on computer B access a file in computer A?? maybe a group of users, like admins... can that be done? when im adding permisions i see i can select the location (i guess that is a network location and i can change that to the name of another comp in the workgroup) but wht i click to change it it only appears the computer im woking in... i mean if i do that from comp A it only appears the name of comp A! if that cant be done what are other posibilites son not all ppl in the network can se the files? passwords? other permissions?
hope you can helop me please and sorry again if i confuse you but i need to go =P thxxx again