Have you read the Security FAQ document there was a link to?
http://support.microsoft.com/support/access/content/secfaq.aspIn Access there is a "User-Level security Wizard" its found in the tools+security menu. But it would be best to read a little about how it works before jumping in!
Basically you give permissions to Groups, each group can have different permissions to the dB. Then the users who each has their own password gets assigned to one or more of these groups. So moving a user to another group gives him/her the permissions for that group.
Access uses a file named system.mdw for security and this file MUST follow the application so that all users have access to the same .mdw file. Actually security is always ON and there is a file being used named system.mdw. So if you dont tell Access which .mdw to use then it uses the default.