You are on the right track, you say you have enabled file and print sharing and have selected a folder for sharing.
Two things to look at are "Sharing Permissions" and "Security" both can be accessed by right clicking on the folder and selecting either "Sharing and security" or "Properties".
This is what I would do; first make sure that all the users’ names on one computer are also on the other computer(s). For example if you had a user called "bob" on one computer create a new account on the other computer also called "bob". (Even if they don’t use that computer)
Also create passwords for all users and keep the passwords for each user the same on both computers.
Once that is done then go back into the "Sharing Permissions" and make sure that the users you wish to have access to the folder are allowed.
Do the same with the file security.
I would also disable windows firewall on the network card that you are using on both computers and check if any other firewall or security software is running, if so configure it to allow file and print sharing.