In the Finder, navigate to the Users folder (its one level down from the top level of your startup disk). You should see your own home folder (with a house icon), and the home folder of the other user account whose files you wish to have access to.
Highlight the other user's home folder, and choose Get Info from the File menu (or hit Command-I).
At the bottom of the Info panel that pops up, you will see a list of Permissions (if you don't see it, expand the section titled Sharing & Permissions), and a tiny padlock icon. Click on the padlock icon.
You will be challenged to enter your password. Enter it and continue.
The padlock icon should now be unlocked. Click the plus button at the bottom of the info pane. Your user name will be added to the list under Sharing and Permissions. Click on the Privilege next to it, and adjust it to whatever level of access you would like to have (Read Only, Read & Write, etc.) Next, click the box with a gear in it at the bottom, and choose Apply to enclosed items. You will be warned that this is an irreversible process. Click OK, and progress bar will drop down. This step can take some time, depending on how many files and folder are inside the other user's home directory.
When the progress bar disappears, you are done. Close the info panel. You can now open up and browse the other user's home folder as if it were your own.
This whole thing is possible because of something called Access Control Lists. From the finder, you can adjust the access of any file or folder on a per-use bases without changing the ownership or group membership of these items. ACLs are incredibly flexible, but to leverage their full potential you have to jump into the Terminal. ACLs allow you control on a per-user or per-group basis access rules such as:
- Ability to delete.
- Ability to read and/or change extended attributes
- Ability to read and/or change security information
- Ability to change ownership
- Ability to read the contents of a folder
- Ability to add and/or remove items from a folder
- Ability to read, write, append, or execute (run) an item.